diffoscope: fix nixpkgs patch to be black-compliant
Otherwise diffocsope's tests fail as:
diffoscope> - LINKS_RE = re.compile(r'Links: [0-9]+\s+')
diffoscope> + LINKS_RE = re.compile(r"Links: [0-9]+\s+")
diffoscope> FAILED tests/test_source.py::test_code_is_black_clean