Update docs to remove 3.6 from supported versions

[johnc1231]

Follow up to #11206. Python 3.6 is EOL, so let's not claim to support it or encourage its use anymore.

[daniel-goldstein]

hail/python/setup.py: python_requires=">=3.6",
hail/python/hail/backend/spark_backend.py: raise EnvironmentError('Hail with spark {} requires Python 3.6 or 3.7, found {}.{}'.format(
hail/python/requirements.txt:orjson==3.6.4
hail/python/hailtop/batch/batch.py: 3.6, 3.7, or 3.8.
hail/python/hailtop/batch/docker.py: f'Python versions other than 3.6, 3.7, or 3.8 (you are using {major_version}.{minor_version}) are not supported')
hail/python/hailtop/batch/batch_pool_executor.py: include Python 3.6 or later and must have the dill Python package
hail/python/hailtop/batch/batch_pool_executor.py: f'You must specify an image if you are using a Python version other than 3.6, 3.7, or 3.8 (you are using {version})')
hail/python/hailtop/batch/docs/cookbook/random_forest.rst:choose a suitable image for you if your Python version is 3.6, 3.7, or 3.8.
hail/python/hailtop/batch/docs/docker_resources.rst:exists to base your image on. An image that has Python preinstalled is python:3.6-slim-stretch and
hail/python/hailtop/batch/docs/tutorial.rst:f-strings were added to Python in version 3.6 and are denoted by the 'f' character
hail/python/hailtop/batch/backend.py: f"You must specify 'image' for Python jobs if you are using a Python version other than 3.6, 3.7, or 3.8 (you are using {version})")
docker/python-dill/push.sh:for version in 3.6 3.6-slim 3.7 3.7-slim 3.8 3.8-slim
docker/third-party/images.txt:python:3.6
docker/third-party/images.txt:python:3.6-slim
benchmark/python/setup.py: python_requires=">=3.6",

Found a few more invocations of 3.6. I'm not sure when you want to update the python_requires and makes sense to leave the 3.6 docker images around for a little while, but the other places in the batch documentation and warnings seem a good thing to update now too.

[johnc1231]

I think this covers most spots. I updated the python_requires, left the docker images, tried to update batch docs.

[daniel-goldstein]

If the > symbol there is to be believed should we require 3.7 or 3.8? Or should that be >=?

[daniel-goldstein]

If it's >= then maybe that fix is out of scope here since that would be breaking?

[johnc1231]

Huh, this must not actually run, since we definitely allow users to use 3.8. I'll investigate, good catch.

[johnc1231]

Oh wait, I remember now. This is because Spark 2 can't use > 3.8. So yeah, 3.7 is the only ok version, that's fine. I plan on dropping Spark 2 support shortly anyway.