Python compat path removal based on Django’s PY* constants

[adamchainz]

Description

Django has private constants for Python version:

https://github.com/django/django/blob/84206607d6bfd61e7f7a88b51163ffd4153e3b5a/django/utils/version.py#L9-L18

We could copy pyupgrade’s old version removal feature for these constants.

Taking an extra argument with the target Python version might be a bit of feature creep. Perhaps we could instead take the minimum supporte Python version from the target Django version (each Django version’s release notes lists the supported Python version, we’d need that data in django-upgrade).