tests: enable import checker for all python files (including no .py files)
i18n/posplit is excluded as it couldn't be trivially fixed. That's the same
as
99a2bdad0fda.
tests: make 'f' utility import hashlib unconditionally
It must exist on Python 2.5+.
tests: remove unused import from 'f' utility
It should have been caught by pyflakes.