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 -*-

[DEFAULT]
replvalue1 = in the root namespace -- too

[config]
key1 = in the root namespace
key2 = %(replvalue1)s

[config.tree1]
key3 = :int:0x20

[config.tree1.tree2]
key4 = get this as `tree1.tree2.key4'
key5 = :bool:TRUE
key6 = off