base: pin pyflakes

Merged Anton Shestakov requested to merge topic/v1.0/pin-pyflakes into branch/v1.0

The new 3.0.0 tries to check blocks guarded by TYPE_CHECKING, and not always successfully. Maybe some time later we can upgrade, but for now newer pyflakes generates a lot of noise about unused imports (because in many cases we still write type hints as comments and pyflakes is not handling them).

No need to pin it on Python 2 (and we can't anyway) because 2.4.0 was the last release of pyflakes that supported it. 2.5.0 claims to be Python 3-only.

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