Remove 3.6 compat code

Python 3.6 is the last Python version for which we have notable compat code still around.<br><br>Doesn't include:

* contextvars [https://github.com/getsentry/sentry-python/issues/5037](https://github.com/getsentry/sentry-python/issues/5037)
* datetime_from_isoformat [https://github.com/getsentry/sentry-python/issues/5009](https://github.com/getsentry/sentry-python/issues/5009)