do not suppress --no-input argument

[cs01]

This PR removes suppression of the --no-input argument, which has existed in pip since 2010.

Unit tests already exist for this argument.

pip/tests/unit/test_options.py

Lines 341 to 345 in 3f4bb75

	def test_no_input(self):
	options1, args1 = main(['--no-input', 'fake'])
	options2, args2 = main(['fake', '--no-input'])
	assert options1.no_input
	assert options2.no_input

After 5 years, nobody has raised any objections on #2429 for why this option is hidden. I recently was made aware of the --no-input flag, and would never have known about it had @chrahunt not mentioned it. I plan to use it in pipx to resolve pypa/pipx#219.

[chrahunt]

I don't see any functional tests that check:

1. we prompt as expected when hitting an endpoint that demands auth (returns 401)
2. we don't prompt in the same situation when --no-input is provided on the command line

It would be a good idea to have these before we "officially" support the flag, to avoid regressions.

We can probably put something together that returns 401 when needed (and verifying that it was called) using the server test helper. We may want to pull in e.g. pexpect for the interactive part of the positive test case.

[cs01]

Unfortunately I'm afraid I don't have the time to dedicate to set up more functional tests. Let me know if you'd like me to withdraw this PR, or if you'd like to keep it open and push to it (edits are allowed from maintainers).

[chrahunt]

No worries. I created #7688 to track adding docs/tests for this, so I'll close this now. 👍