Introduce pyupgrade

Introduce pyupgrade hook:
Issue with await in f-string:
- With Py3.6 checks as minimum: Not possible, requires Py3.7.
- https://github.com/asottile/pyupgrade/issues/481
- Run with --py36-plus, remove await in f-string changes, run hook with --py3-plus.
- Fixed with pyupgrade v2.20.1, v2.21.0
Apply pyupgrade to Python files in silkaj and tests folders
Add pyupgrade job based on pre-commit

Edited Jul 13, 2021 by Moul

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