tests/test-gendoc-ru.t

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.

This is a graft of a similar fix that ended up on default.

Differential Revision: https://phab.mercurial-scm.org/D8628

#require docutils gettext

  $ $TESTDIR/check-gendoc ru
  checking for parse errors