Drop Python 3.9 support, require 3.10+

[EwoutH]

Following SPEC 0 Python 3.9 support is dropped for future minor and major releases, and will require Python 3.10+ from version 3.0 onwards.

The main benefit of requiring Python 3.10 is that we can implement type hinting more elegant in the workbench.

The 2.5.x branch will keep supporting Python 3.9.