Add a comment showing how to call main using runpy

[pfmoore]

Context: https://discuss.python.org/t/can-we-finally-add-a-minimal-api-to-pip/2833/14

This does not endorse using main() as an API to pip.

The only purpose of this comment is to document the correct invocation of runpy.run_module to execute pip, for those people who decide to run pip in-process in spite of our advice.

This would helps wrapper scripts like ensurepip, to protect them against needing to change if we move main() again.

[pradyunsg]

Context for those wondering: https://discuss.python.org/t/can-we-finally-add-a-minimal-api-to-pip/2833

[pradyunsg]

nit: some blank lines have # at the start, other don't.

I don't really want to push for >5 hours of cumulative CI runtime, for something this trivial tho.

[pfmoore]

Blast, I missed those blank lines. Agreed re the CI costs. I wonder if there's a way to set things up so we can mark certain commits as "don't run CI"? But that's a topic for another issue.

Thanks for the review and the improved wording 🙂

[pradyunsg]

"don't run CI"

You can add [skip ci] or [ci skip] to commits messages -- all CI services skip builds on those commits.

I don't know how they get reported to GitHub's Checks tho. :(