[FreeThreading] test_ssl fails with env_changed: warnings.filters was modified by test_ssl

[vstinner]

On a Free Threaded build, test_ssl fails with:

$ ./python -m test test_ssl --fail-env-changed -m test_ssl_in_multiple_threads 
(...)
Warning -- warnings.filters was modified by test_ssl
Warning --   Before: (2199023321584, [], [])
Warning --   After:  (2199997062288, [('always', None, <class 'ResourceWarning'>, None, 0)], [('always', None, <class 'ResourceWarning'>, None, 0)]) 
test_ssl failed (env changed)
(...)
Result: ENV CHANGED

The problem comes from test_check_hostname_idn() which uses warnings_helper.check_no_resource_warning() (changes warnings filters).

[colesbury]

To add to your description: check_no_resource_warning() uses warnings.filterwarnings and warnings.catch_warnings, which are not thread-safe in Python (even with the GIL).

So it's not safe to run test_check_hostname_idn from multiple threads concurrently like test_ssl_in_multiple_threads is doing.