bpo-30681: Support invalid date format or value in email Date header (GH-22090)

I am re-submitting an older PR which was abandoned but is still relevant, #10783 by @timb07.

The issue being solved () is still relevant. The original PR #10783 was closed as
the final request changes were not applied and since abandoned.

In this new PR I have re-used the original patch plus applied both comments from the review, by @maxking and @pganssle.


For reference, here is the original PR description:
In email.utils.parsedate_to_datetime(), a failure to parse the date, or invalid date components (such as hour outside 0..23) raises an exception. Document this behaviour, and add tests to test_email/test_utils.py to confirm this behaviour.

In email.headerregistry.DateHeader.parse(), check when parsedate_to_datetime() raises an exception and add a new defect InvalidDateDefect; preserve the invalid value as the string value of the header, but set the datetime attribute to None.

Add tests to test_email/test_headerregistry.py to confirm this behaviour; also added test to test_email/test_inversion.py to confirm emails with such defective date headers round trip successfully.

This pull request incorporates feedback gratefully received from @bitdancer, @brettcannon, @Mariatta and @warsaw, and replaces the earlier PR #2254.

Automerge-Triggered-By: GH:warsaw

Lib/email/errors.py

@@ -108,3 +108,6 @@ class NonASCIILocalPartDefect(HeaderDefect):
     """local_part contains non-ASCII characters"""
     # This defect only occurs during unicode parsing, not when
     # parsing messages decoded from binary.
+
+class InvalidDateDefect(HeaderDefect):
+    """Header has unparseable or invalid date"""