ref(py3): Make PY3 the default*, add SENTRY_PYTHON (#763)

This is in preparation to make the PY3 version the default* for Docker images and self-hosted. It is part 2/5:

1. Add -py2 variants for the Python 2 build tags and introduce the SENTRY_PYTHON2 env variable usage (getsentry/sentry#22460)
2. Switch getsentry/onpremise to Python 3 by default*, introducing the SENTRY_PYTHON2 env var for Py2 builds via the -py2 suffix
3. Move the unsuffixed version of the builds to Python 3
4. Remove the SENTRY_PYTHON3 env var support and -py3 prefix usage from getsentry/onpremise
5. Remove tagging of -py3 builds from here

_* this will only happen when item 3 above gets landed_