Update pyupgrade's target version to Python 3.9

[ulgens]

Description

Updates target-version parameter for pyupgrade to 3.9 from 3.8, and applies changes coming with the change.

Checklist:

• I have added the relevant tests for this change.
  • The changes don't remove or add functionality. I'm not sure if there is anything to be tested with the changes.
• I have added an item to the Pending section of docs/changes.rst.
  • The changes only relate to code quality in the repo and none of the features, again I'm not sure if there is anything to record in to the change log.

[matthiask]

Thank you! Great work.