gh-143959: Fix test_datetime if _datetime is unavailable

[serhiy-storchaka]

• Issue: test_sys globally requires _datetime for cpython_only test #143959

[vstinner]

I tested this change with this local patch:

diff --git a/Lib/test/test_datetime.py b/Lib/test/test_datetime.py
index 005187f13e6..013816e99b3 100644
--- a/Lib/test/test_datetime.py+++ b/Lib/test/test_datetime.py@@ -4,6 +4,7 @@
from test.support.import_helper import import_fresh_module
+sys.modules['_datetime'] = None
TESTS = 'test.datetimetester'

On the main branch, test_datetime fails with:

test test_datetime crashed -- Traceback (most recent call last):
File "/home/vstinner/python/main/Lib/test/libregrtest/single.py", line 210, in _runtest_env_changed_exc
_load_run_test(result, runtests)
~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
File "/home/vstinner/python/main/Lib/test/libregrtest/single.py", line 165, in _load_run_test
regrtest_runner(result, test_func, runtests)
~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/vstinner/python/main/Lib/test/libregrtest/single.py", line 118, in regrtest_runner
test_result = test_func()
File "/home/vstinner/python/main/Lib/test/libregrtest/single.py", line 162, in test_func
return run_unittest(test_mod, runtests)
File "/home/vstinner/python/main/Lib/test/libregrtest/single.py", line 38, in run_unittest
raise Exception("errors while loading tests")
Exception: errors while loading tests

With this PR, test_datetime pass.

[vstinner]

LGTM

[miss-islington-app]

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

[bedevere-app]

GH-146288 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-146289 is a backport of this pull request to the 3.13 branch.

[serhiy-storchaka]

Thank you @vstinner for your review and for your patch #143997 (comment) which I used to write and test these changes.