get-pip: add notes to docs for specifying versions

[ewdurbin]

get-pip.py supports installing specified versions of pip, wheel, setuptools... and any dependency for that matter, but this is not clearly exposed in the documentation at https://pip.pypa.io/en/stable/installing/#installing-with-get-pip-py

[pradyunsg]

@ewdurbin I'll investigate the CI failures over the weekend. If you want to, I'm more than happy to defer.

Otherwise, could you make a PR from your fork instead? I'm pretty sure they won't fail.

[xavfernandez]

This seems good to go.
@ewdurbin could you rebase on master to retrigger the builds ?

[lock]

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