Add an optional C-implementation for configmix.config.unquote and configmix.config.pathstr2path. This is currently for Python 3.5+. It is tested with Python 3.7 and Python3.8 (FreeBSD 12.2 amd64, LLVM 10.0.1). A build for the stable API ("abi3") fails because PyUnicode_New() is currently not in the stable API. Also includes are extended tests for unquote() and pathstr2path().

{"tmpdir": "{{_tmpdir}}\\3",
 "db": {
     "user": {
	 "name": "the_database_user_2",
	 "__doc2": "comment",
	 "pwd": "the-database-password-2"
     },
     "locinfo": {
	 "ro": {
	     "hostname": "3rd-host"
	 }
     }
 }
}