Fix graphite.util.epoch for Django 5

[samestep]

This PR fixes the test_epoch_naive failure that currently occurs when running the Graphite test suite with Django 5; as a specific reproducible example, here's the relevant nix-build -A python3Packages.graphite-web log snippet at NixOS/nixpkgs@4060765:

FAIL: test_epoch_naive (tests.test_util.UtilTest.test_epoch_naive)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/nix/store/ccf31a4yhdqra20xb8yjcps9rlp42q5h-python3.13-mock-5.2.0/lib/python3.13/site-packages/mock/mock.py", line 1468, in patched
    return func(*newargs, **newkeywargs)
  File "/build/source/webapp/tests/test_util.py", line 40, in test_epoch_naive
    self.assertEqual(util.epoch(dt), 600)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
AssertionError: 1020 != 600

#2886 added support for Django 5, but this repo's CI didn't catch the test failure because the tox configuration forces Django 4:

graphite-web/.github/workflows/tests.yml

Lines 72 to 83 in b5d272b

	- name: Test with tox
	env:
	TEST_REDIS_HOST: localhost
	TEST_REDIS_PORT: ${{ job.services.redis.ports[6379] }}
	TEST_MYSQL_HOST: localhost
	TEST_MYSQL_PORT: ${{ job.services.mariadb.ports[3306] }}
	TEST_MYSQL_PASSWORD: graphite
	TEST_POSTGRES_HOST: localhost
	TEST_POSTGRES_PORT: ${{ job.services.postgres.ports[5432] }}
	TEST_POSTGRESQL_PASSWORD: graphite
	run: |
	tox

graphite-web/tox.ini

Line 37 in b5d272b

Django>=4.2,<5

The reason the test fails is because of a change in Django 5:

Support for pytz timezones is removed.

Specifically, django/django@e6f8243 removed the pytz-specific branch from django.utils.timezone.make_aware.

The fix is to replace this make_aware call with code equivalent to what the Django 4 implementation of make_aware would have done. Note that the effect of is_dst=None is to raise in some specific cases; otherwise the localize method defaults to is_dst=False, which would silently use standard time in those cases.

The other make_aware call (in the epoch_to_dt function) does not need to change because the UTC timezone is unaffected by this Django behavior change.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.90%. Comparing base (5df4cd4) to head (a2fdc90).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2914      +/-   ##
==========================================
+ Coverage   76.80%   76.90%   +0.09%     
==========================================
  Files          88       88              
  Lines        9700     9703       +3     
  Branches     1805     1806       +1     
==========================================
+ Hits         7450     7462      +12     
+ Misses       1983     1971      -12     
- Partials      267      270       +3     

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