Remove 3.6 compat code

[sentrivana]

Python 3.6 is the last Python version for which we have notable compat code still around.

Doesn't include:

• contextvars Remove contextvars compat #5037
• datetime_from_isoformat Remove datetime_from_isoformat #5009

[linear]

PY-1954

[ulgens]

Is #5005 a prerequisite for this issue?

[sentrivana]

#5005 is the branch where we'll be working on the new major, so this issue will be implemented on that branch.