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().

# -*- coding: utf-8 -*-

key1 = "in the root namespace"
key2 = "in the root namespace -- too"

[tree1]
key3 = 0x20

[tree1.tree2]
key4 = "get this as `tree1.tree2.key4'"
key5 = true
key6 = "off"