--- a/setup.py	Sun Jan 09 13:40:22 2022 +0100
+++ b/setup.py	Sun Jan 09 13:46:57 2022 +0100
@@ -62,6 +62,12 @@
     wcp_idx = None
 else:
     del sys.argv[wcp_idx]
+try:
+    pure_only_idx = sys.argv.index("--pure-only")
+except ValueError:
+    pure_only_idx = None
+else:
+    del sys.argv[pure_only_idx]    
 
 
 #
@@ -73,42 +79,72 @@
     print("removing `%s.egg-info'" % (PROJECT_NAME,))
     shutil.rmtree(PROJECT_NAME + ".egg-info")
 
-if wcp_idx is None:
-    #
-    # Handle the optinal C-extension for Python3.7+ and CPython only.
-    # PyPy does not need this.
-    #
+if pure_only_idx is None:   
+    if wcp_idx is None:
+        #
+        # Handle the optinal C-extension for Python3.7+ and CPython only.
+        # PyPy does not need this.
+        #
+
+        # The C-extension uses multi-phase module initialization (PEP 489, PY 3.5+)
+        if (platform.python_implementation() == "CPython"
+            and (sys.version_info[0] > 3
+                 or (sys.version_info[0] == 3 and sys.version_info[1] >= 5))):
 
-    # The C-extension uses multi-phase module initialization (PEP 489, PY 3.5+)
-    if (platform.python_implementation() == "CPython"
-        and (sys.version_info[0] > 3
-             or (sys.version_info[0] == 3 and sys.version_info[1] >= 5))):
+            # The stable API for Python 3.7+ is used
+            if sys.version_info[0] == 3 and sys.version_info[1] < 7:
+                py_limited_api = False
+            else:
+                py_limited_api = True
 
-        # The stable API for Python 3.7+ is used
-        if sys.version_info[0] == 3 and sys.version_info[1] < 7:
-            py_limited_api = False
-        else:
-            py_limited_api = True
+            if py_limited_api:
+                define_macros = [("Py_LIMITED_API", "0x03070000")]
+            else:
+                define_macros = []
+
+            try:
+                from setuptools import Extension
+            except ImportError:
+                from distutils.core import Extension
 
-        if py_limited_api:
-            define_macros = [("Py_LIMITED_API", "0x03070000")]
-        else:
-            define_macros = []
-
-        try:
-            from setuptools import Extension
-        except ImportError:
-            from distutils.core import Extension
+            ext_modules = [
+                Extension(
+                    name="configmix._speedups",
+                    sources=["configmix/_speedups.c"],
+                    define_macros=define_macros,
+                    py_limited_api=py_limited_api,
+                    optional=True
+                ),
+            ]
 
-        ext_modules = [
-            Extension(
-                name="configmix._speedups",
-                sources=["configmix/_speedups.c"],
-                define_macros=define_macros,
-                py_limited_api=py_limited_api,
-                optional=True
-            ),
-        ]
+            if py_limited_api:
+                #
+                # Build a wheel that is properly named using the stable API
+                #
+                try:
+                    import wheel.bdist_wheel
+                except ImportError:
+                    pass
+                else:
+                    class BDistWheel(wheel.bdist_wheel.bdist_wheel):
+                        def finalize_options(self):
+                            # Synchronize this with Py_LIMITED_API
+                            self.py_limited_api = 'cp37'
+                            super().finalize_options()
+
+                    cmdclass["bdist_wheel"] = BDistWheel
+    else:
+
+        if not os.path.isfile("configmix/_speedups.pyd"):
+            raise RuntimeError("no _speedups.pyd found")
+
+        setup_extra_kwds["package_data"] = {
+            "configmix": ["*.pyd"]
+        }
+
+        ext_modules = []
+
+        py_limited_api = True
 
         if py_limited_api:
             #
@@ -121,62 +157,38 @@
             else:
                 class BDistWheel(wheel.bdist_wheel.bdist_wheel):
                     def finalize_options(self):
-                        # Synchronize this with Py_LIMITED_API
+                        #
+                        # Synchronize this with Py_LIMITED_API and with the
+                        # external build of _speedups.pyd.
+                        # Also use the --plat-name (-p) for tagging the Wheel
+                        # properly (e.g. -p win_amd64).
+                        #
                         self.py_limited_api = 'cp37'
                         super().finalize_options()
 
                 cmdclass["bdist_wheel"] = BDistWheel
-else:
 
-    if not os.path.isfile("configmix/_speedups.pyd"):
-        raise RuntimeError("no _speedups.pyd found")
+        from setuptools.dist import Distribution
 
-    setup_extra_kwds["package_data"] = {
-        "configmix": ["*.pyd"]
-    }
-
-    ext_modules = []
-
-    py_limited_api = True
-
-    if py_limited_api:
         #
-        # Build a wheel that is properly named using the stable API
+        # Force a binary package. An empty ext_modules does not do this always.
+        # Tested with wheel v0.29.0
         #
-        try:
-            import wheel.bdist_wheel
-        except ImportError:
-            pass
-        else:
-            class BDistWheel(wheel.bdist_wheel.bdist_wheel):
-                def finalize_options(self):
-                    #
-                    # Synchronize this with Py_LIMITED_API and with the
-                    # external build of _speedups.pyd.
-                    # Also use the --plat-name (-p) for tagging the Wheel
-                    # properly (e.g. -p win_amd64).
-                    #
-                    self.py_limited_api = 'cp37'
-                    super().finalize_options()
+        class BinaryDistribution(Distribution):
+            """Distribution which always forces a binary package with
+
+            platform name
 
-            cmdclass["bdist_wheel"] = BDistWheel
-
-    from setuptools.dist import Distribution
+            """
+            def has_ext_modules(foo):
+                return True
 
-    #
-    # Force a binary package. An empty ext_modules does not do this always.
-    # Tested with wheel v0.29.0
+        setup_extra_kwds["distclass"] = BinaryDistribution
+else:
     #
-    class BinaryDistribution(Distribution):
-        """Distribution which always forces a binary package with
-
-        platform name
-
-        """
-        def has_ext_modules(foo):
-            return True
-
-    setup_extra_kwds["distclass"] = BinaryDistribution
+    # pure
+    #
+    pass
 
 
 if ext_modules is not None: