tests: add flake8 check
This test will hopefully help prevent style errors. To be determined:
which errors / warnings to ignore?
#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:**.py' | xargs -0 flake8 || true