tests: skip pyflakes for mercurial/thirdparty/
The current version of pyflakes (2.2.0) correctly detects one issue:
mercurial/thirdparty/selectors2.py:335:40 '...'.format(...) has unused arguments at position(s): 1
But we're not interested in fixing lint errors in third-party code, so we need
to exclude at least selectors2.py. And in the discussion for this patch it was
decided to just skip the entire thirdparty directory.
Differential Revision: https://phab.mercurial-scm.org/D8619