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

tests/test-check-pyflakes.t

@@ -16,9 +16,7 b' run pyflakes on all tracked files ending'
   $ testrepohg locate 'set:**.py or grep("^#!.*python")' \
   > -X hgext/fsmonitor/pywatchman \
   > -X mercurial/pycompat.py -X contrib/python-zstandard \
-  > -X mercurial/thirdparty/cbor \
-  > -X mercurial/thirdparty/concurrent \
-  > -X mercurial/thirdparty/zope \
+  > -X mercurial/thirdparty \
   > 2>/dev/null \
   > | xargs $PYTHON -m pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
   contrib/perf.py:*: undefined name 'xrange' (glob) (?)