Deprecate naive datetime in StreamResponse.last_modified setter

[amitmishra11]

What do these changes do?

Emit a DeprecationWarning when a naive datetime.datetime is passed
to StreamResponse.last_modified. Previously, naive datetimes were
silently accepted, but utctimetuple() on a naive datetime returns the
local time fields unchanged, which then gets labelled as GMT in the
Last-Modified HTTP header. This means that calling
resp.last_modified = datetime.now() (which returns local time) would
produce an incorrect header value -- the local time would be stored
verbatim as if it were UTC.

The fix adds a clear deprecation warning directing users to timezone-aware
datetimes (e.g. datetime.now(datetime.timezone.utc)).

Are there changes in behavior for the user?

Yes: passing a naive datetime now emits a DeprecationWarning. The
actual value stored in the header is unchanged from the previous
behaviour. A future release can raise a TypeError or ValueError for
naive datetimes once the deprecation period has elapsed.

Is it a substantial burden for the maintainers to support this?

No. It is a one-line guard plus a warning in one setter, with a single
new test. The warning text is self-explanatory and points directly to
the fix.

Related issue number

Fixes #5304

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES/ folder

Test output

============================= test session starts =============================
collected 10 items

tests/test_web_response.py::test_last_modified_initial PASSED
tests/test_web_response.py::test_last_modified_string PASSED
tests/test_web_response.py::test_last_modified_timestamp PASSED
tests/test_web_response.py::test_last_modified_datetime PASSED
tests/test_web_response.py::test_last_modified_datetime_naive_warns PASSED
tests/test_web_response.py::test_last_modified_reset PASSED
tests/test_web_response.py::test_last_modified_invalid_type PASSED
tests/test_web_response.py::test_last_modified_string_invalid[xxyyzz] PASSED
tests/test_web_response.py::test_last_modified_string_invalid[...] PASSED
tests/test_web_response.py::test_last_modified_string_invalid[...] PASSED
===================== 10 passed in 0.29s ==============================

Drafted with Claude Sonnet 4.6; reviewed by amitmishra11.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.96%. Comparing base (9d183e3) to head (6d81eca).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #13047   +/-   ##
=======================================
  Coverage   98.96%   98.96%           
=======================================
  Files         131      131           
  Lines       48156    48163    +7     
  Branches     2499     2500    +1     
=======================================
+ Hits        47656    47663    +7     
  Misses        376      376           
  Partials      124      124           

Flag	Coverage Δ
Autobahn	22.21% <14.28%> (-0.01%)	⬇️
CI-GHA	98.90% <100.00%> (-0.01%)	⬇️
OS-Linux	98.67% <100.00%> (+<0.01%)	⬆️
OS-Windows	97.04% <100.00%> (+<0.01%)	⬆️
OS-macOS	97.95% <100.00%> (-0.01%)	⬇️
Py-3.10	98.14% <100.00%> (-0.01%)	⬇️
Py-3.11	98.41% <100.00%> (+<0.01%)	⬆️
Py-3.12	98.50% <100.00%> (+<0.01%)	⬆️
Py-3.13	98.47% <100.00%> (+<0.01%)	⬆️
Py-3.14	98.49% <100.00%> (+<0.01%)	⬆️
Py-3.14t	97.58% <100.00%> (-0.01%)	⬇️
Py-pypy-3.11	97.45% <100.00%> (-0.01%)	⬇️
VM-macos	97.95% <100.00%> (-0.01%)	⬇️
VM-ubuntu	98.67% <100.00%> (+<0.01%)	⬆️
VM-windows	97.04% <100.00%> (+<0.01%)	⬆️
cython-coverage	38.12% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

Merging this PR will not alter performance

✅ 83 untouched benchmarks
⏩ 83 skipped benchmarks¹

---

_{Comparing amitmishra11:fix/warn-naive-datetime-last-modified (6d81eca) with master (9d183e3)²}

Footnotes

1. 83 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on master (e3774b4) during the generation of this report, so 9d183e3 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩