Sync .python-version with upstream sentry

Seems like our committed version isn't quite the same as upstream, so we could fetch it from https://github.com/getsentry/sentry/blob/master/.python-version instead.

We might be able to do this the same way as https://github.com/siemens/sentry-auth-oidc/blob/master/Makefile#L11.

/cc @dlouzan 