tests: consistently use pyflakes as a Python module
We check availability of pyflakes as a module, and also running it for real as
a module. Only fair to test filterpyflakes.py working correctly when using
pyflakes as a module too.
Differential Revision: https://phab.mercurial-scm.org/D8617
#require docutils gettext
$ $TESTDIR/check-gendoc pt_BR
checking for parse errors