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; -*-

__comment1 = "Comment 1"
key1 = "the value"
key2 = 2
key3 = 5.7
key4 = true
key5 = false
key6 = 0o377
__comment2 = "Comment no 2"
key7 = "Umlaute: ÄÖÜäöüß"