bpo-40275: Add warnings_helper submodule in test.support

[shihai1991]

move the following functions, classes to warnings_helper:

• check_syntax_warning()
• ignore_warnings()
• WarningsRecorder
• check_warnings(), _filterwarnings()
• check_no_warnings()
• check_no_resource_warning()

https://bugs.python.org/issue40275

[shihai1991]

@vstinner Hi, victor. Pls take a look if you have free times. Thanks.

[vstinner]

In support/init.py, "import warnings" is only used by _MemoryWatchdog. Can you please move the import inside _MemoryWatchdog.start()? That's the whole purpose of https://bugs.python.org/issue40275 : remove imports.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[shihai1991]

In support/init.py, "import warnings" is only used by _MemoryWatchdog. Can you please move the import inside _MemoryWatchdog.start()? That's the whole purpose of https://bugs.python.org/issue40275 : remove imports.

make sense, done.

[vstinner]

LGTM.

[shihai1991]

LGTM.

thanks, victor.