ref(py3): Remove SENTRY_PYTHON3 and -py3 versons

[BYK]

This is in preparation to make the PY3 version the default for Docker images and self-hosted. It is part 4/5:

1. Add -py2 variants for the Python 2 build tags and introduce the SENTRY_PYTHON2 env variable usage (ci(py3): Make py2 builds available under -py2 suffix sentry#22460)
2. Switch getsentry/onpremise to Python 3 by default*, introducing the SENTRY_PYTHON2 env var for Py2 builds via the -py2 suffix (ref(py3): Make PY3 the default*, add SENTRY_PYTHON #763)
3. Move the unsuffixed version of the builds to Python 3 (build(py3): Make PY3 images default (w/o suffix) sentry#22466)
4. Remove the SENTRY_PYTHON3 env var support and -py3 prefix usage from getsentry/onpremise
5. Remove tagging of -py3 builds from getsentry/sentry

[chadwhitacre]

$ gb
  byk/ref/bye--py3
* master
$ ag py3
install.sh
220:docker pull ${SENTRY_IMAGE}${SENTRY_PYTHON2:+-py2}${SENTRY_PYTHON3:+-py3} || true;

sentry/Dockerfile
4:FROM ${SENTRY_IMAGE}${SENTRY_PYTHON2:+-py2}${SENTRY_PYTHON3:+-py3}
$ ag PYTHON3
install.sh
220:docker pull ${SENTRY_IMAGE}${SENTRY_PYTHON2:+-py2}${SENTRY_PYTHON3:+-py3} || true;

docker-compose.yml
11:      - SENTRY_PYTHON3

sentry/Dockerfile
3:ARG SENTRY_PYTHON3
4:FROM ${SENTRY_IMAGE}${SENTRY_PYTHON2:+-py2}${SENTRY_PYTHON3:+-py3}
$     
$ 
$ 
$ co byk/ref/bye--py3  
Switched to branch 'byk/ref/bye--py3'
Your branch is up to date with 'origin/byk/ref/bye--py3'.
$ ag py3
$ ag PYTHON3
$