tests: run pyflake on all python files
We have fixed the handfull of error pyflakes found on evolve so we can setup
the test to prevent regression. Next stop flake8.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate 'set:**.py or grep("^!#.*python")' 2>/dev/null \
> | xargs pyflakes 2>/dev/null
run flake8 if it exists; if it doesn't, then just skip
$ type flake8 >/dev/null 2>/dev/null && hg files -0 'glob:hgext3rd/topic/**.py' | xargs -0 flake8 || true