Do not use `utc_now` on module level (#372)

* Do not use `utc_now` on module level

Looks like that this value is never used anywhere, and it should not be, because it is created once on a module import time.

I found this while working on https://github.com/python/typeshed/pull/13514

* Fix tests as well

* fix lint import issue

* try to fix black lint errors

* fix again

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Co-authored-by: Asher Foa <asher@userclouds.com>