Fix list of ignored problems for flake8

[menshikh-iv]

Now we using --ignore=E501,E731,E12,W503,E402 parameters for flake8, we can be more "strict" and reduce list of "ignored" checks

What this checks means (full list here)

Code	Message	Action (from us)
E501	line too long (>80 characters)	remove (but need track size with adequate limit, like 120)
E731	do not assign a lambda expression, use a def	remove
W503	line break occurred before a binary operator	-
E402	module level import not at top of file	remove
E12	Many checks ...	remove (need to think about E121, E123, E126)

Related #1636

[menshikh-iv]

For now, need to think about E12 group and (more important) make extended-E501.

[menshikh-iv]

Last part fixed in #1721