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; mode: yaml; indent-tabs-mode: nil; -*-
%YAML 1.1
---

__comment1: Comment 1
key1: the value
key2: 2
key3: 5.7
key4: true
key5: false
key6: 0377
__comment2: Comment no 2
key7: 'Umlaute: ÄÖÜäöüß'