fix: feature-detect getaddresses(strict=) so address parsing works on Python < 3.13

[apoorvdarshan]

Summary

mailparser.utils.get_addresses unconditionally calls
email.utils.getaddresses([raw], strict=True). The strict keyword only
exists on Python 3.13+ (and later 3.9-3.12 security patch releases). Since
mail-parser targets requires-python >=3.9,<3.15, on an earlier patch release
this raises a TypeError and breaks address parsing for every message.

Reported downstream in domainaware/parsedmarc#808 ("getmessages fails with
invalid parameter 'strict'"), where valid messages were consequently moved to
an archived/invalid folder instead of being processed.

Root cause

src/mailparser/utils.py (previously line 137):

parsed = email.utils.getaddresses([raw_header], strict=True)

strict was added to email.utils.getaddresses in CPython 3.13 and backported
only to later 3.9-3.12 security patch releases (e.g. 3.11.10). On an earlier
interpreter such as CPython 3.11.3 the parameter does not exist, so passing it
raises:

TypeError: getaddresses() got an unexpected keyword argument 'strict'

Reproduction

On an interpreter whose email.utils.getaddresses predates strict (simulated
here by patching in the pre-3.13 signature):

import email.utils
from unittest.mock import patch
from mailparser.utils import get_addresses

_orig = email.utils.getaddresses
def legacy_getaddresses(fieldvalues):   # pre-3.13 signature: no `strict`
    return _orig(fieldvalues)

with patch("mailparser.utils.email.utils.getaddresses", new=legacy_getaddresses):
    print(get_addresses("Plain Name <plain@example.com>"))

Actual (before the fix):

TypeError: getaddresses() got an unexpected keyword argument 'strict'

Corrected (after the fix):

[('Plain Name', 'plain@example.com')]

Fix

Feature-detect the strict parameter before passing it, in a small helper
_getaddresses:

if "strict" in inspect.signature(email.utils.getaddresses).parameters:
    return email.utils.getaddresses(fieldvalues, strict=True)
return email.utils.getaddresses(fieldvalues)

The change is localized to get_addresses: it preserves strict, CVE-2023-27043
hardened parsing on interpreters that support it (the existing empty-address
regex fallback is unchanged) and only omits the keyword on older interpreters
where it is unavailable. No behavioral change on Python 3.13+.

Tests

Added tests/test_utils.py::TestUtilsEdgeCases::test_get_addresses_without_strict_parameter,
which patches in a pre-3.13 getaddresses (a real function without strict,
so the feature detection observes the older signature) and asserts the address
is parsed rather than crashing.

• On unmodified code this test fails with the reported TypeError at
  src/mailparser/utils.py:137.
• With the fix it passes.
• Full suite: 211 passed (python -m pytest tests/).
• ruff check and ruff format --check are clean on the changed files.

Disclosure: prepared with AI assistance; reviewed and verified locally.