Tracking: Drop support for python 3.8

[dangotbanned]

Had quite the ordeal trying to avoid the bugs of Protocol in (#2064 (comment)).
If that compatibility code gets merged, I'd really like to remove it ASAP.
It adds a lot of complexity for a version almost 5 months past EOL

I collected a lot of info a while back in a similar issue for altair

• Preparing for Python 3.8 EOL vega/altair#3497

Downstream "<=3.8"

All projects listed in README.md#used-by have dropped 3.8 support, except:

• plotly
  • Python 3.8 support plotly/plotly.py#5054
• scikit-lego
  • Consider dropping python 3.8 tests koaning/scikit-lego#734
• timebasedcv