Support for shorter --python-version=3.7 argument: --3.7 or --python=3.7

[2-5]

I often create venvs for multiple Python versions, --python-version=3.7 is just too long, consider (additional) shorter options:

--3.7
--py=3.7
--python=3.7

[zanieb]

I don't think we'll support --3.7 as that's ambiguous, but we could add a shorter flag for the version.

[dhruvmanila]

--python seems reasonable, it's also being used by pipenv.

[zanieb]

We'll need to be careful that the flag makes sense with #1396

[charliermarsh]

Most commands support -p which I think is fine.