Deceptive instructions for installing Astropy in CASA

These docs are a bit outdated and have a few problems: http://docs.astropy.org/en/stable/install.html#installing-astropy-into-casa

1. The suggestion of using easy_install should be dropped entirely.
2. Don’t suggest using pip.main from a Python prompt. The pip module was never intended to be used as a library and the fact that people do so has been a headache for the pip developers. pip.main has since been deprecated and removed (actually moved to pip._internal) to discourage its use.
3. I have played with Astropy in CASA before but not in a long time, so I don’t know why it’s easier to ask users to do things this way. But if it must be (as opposed to just providing the right python -m pip ... command), something like this would be better:

import subprocess, sys
subprocess.check_call([sys.executable, '-m', 'pip', 'install', ...])

[bsipocz]

but if they don’t willing to support it, then what can we do anyway 🤷‍♀️

[pllim]

There was a stop-gap warning inserted in #8043 for 3.1, so removing milestone.