mypy: fix ci, use a newer flake8 and flake8-pyi, ignore a new error

[hauntsaninja]

The versions are necessary because of some change involving switching from optparse to argparse.

Here are the E741 errors that would be ignored if this was merged.

./mypy/fastparse.py:327:39: E741 ambiguous variable name 'l'
./mypy/fastparse.py:334:35: E741 ambiguous variable name 'l'
./mypy/fastparse.py:1339:35: E741 ambiguous variable name 'l'
./mypy/fastparse.py:1399:39: E741 ambiguous variable name 'l'
./mypy/fastparse2.py:196:35: E741 ambiguous variable name 'l'
./mypy/meet.py:214:13: E741 ambiguous variable name 'l'
./mypy/meet.py:407:20: E741 ambiguous variable name 'l'
./mypy/typeanal.py:1170:13: E741 ambiguous variable name 'l'
./mypy/traverser.py:86:13: E741 ambiguous variable name 'l'
./mypy/report.py:298:46: E741 ambiguous variable name 'l'
./mypy/semanal.py:4524:38: E741 ambiguous variable name 'l'
./mypy/subtypes.py:336:17: E741 ambiguous variable name 'l'
./mypy/subtypes.py:1312:17: E741 ambiguous variable name 'l'
./mypy/test/data.py:383:16: E741 ambiguous variable name 'l'
./mypy/test/data.py:401:32: E741 ambiguous variable name 'l'

[emmatyping]

Thanks!