gh-140482: Preserve and restore stty echo as a test environment

[warsaw]

The test suite sometimes turns off terminal echo after it completes. As far as I can tell, it's completely random and probably caused by tests that don't (or can't!) fully clean up after themselves. After implementing this fix and testing it many times with make test and make quicktest, I see a common set of tests that it happens in, but no real rhyme or reason.

This patch just saves and restores termios.ECHO as a saved test environment and in lots of local testing it works perfectly. Yeah, it's just an annoyance but as #140480 says, this has been annoying me for many releases. Thanks to @zware for suggesting a test environment in my abandoned #140482 PR.

I recommend backporting to all active non-security branches as it will help and there should be no risk involved. One caveat is that I did have to change the signature and semantics of try_get_module() because if the requested module is not already imported, sys.modules[name] will fail, and since termios isn't generally imported, try_get_module() would be unhelpful otherwise. The alternative is to import termios at the top of the module so it's always available, but as no other saved test environment seems to do that, and this environment is global rather than tied to a specific test, I thought this was the better approach. Suggestions welcome.

• Issue: Test suite sometimes leaves terminal in -echo #140482

[zware]

A few style nits and a question, but LGTM.

[miss-islington-app]

Thanks @warsaw for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-140562 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-140563 is a backport of this pull request to the 3.13 branch.

[vstinner]

So which tests fail with this additional check?

[vstinner]

So which tests fail with this additional check?

I reply to myself, Barry already wrote in the issue:

The best I could come up with is test_ssl because that also fails intermittently, and when it fails, it seems to leave the terminal in -echo, but the correlation isn't perfect.

So test_ssl may be the guilty test.

At least, we will know which tests change the terminal to -echo with the new change.

[warsaw]

So test_ssl may be the guilty test.

Just for completeness, it is but other tests also change the environment, e.g. test_remote_pdb