Introduce pyupgrade

• https://github.com/asottile/pyupgrade

Todos

• 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