fix: Fix non-UTC timestamps

[szokeasaurusrex]

Fixes a bug where all datetime timestamps in an event payload were serialized as if they were UTC timestamps, even if they were non-UTC timestamps, completely ignoring the timezone. Now, we convert all datetime objects to UTC before formatting them as a UTC timestamp.

Fixes #3453.

[codecov]

❌ 4 Tests Failed:

Tests completed	Failed	Passed	Skipped
11521	4	11517	1636

View the top 3 failed tests by shortest run time

tests.integrations.aws_lambda.test_aws test_init_error[python3.9]

Stack Traces | 15.8s run time

.../integrations/aws_lambda/test_aws.py:334: in test_init_error
    (event,) = envelope_items
E   ValueError: not enough values to unpack (expected 1, got 0)

tests.integrations.aws_lambda.test_aws test_init_error[python3.10]

Stack Traces | 15.8s run time

.../integrations/aws_lambda/test_aws.py:334: in test_init_error
    (event,) = envelope_items
E   ValueError: not enough values to unpack (expected 1, got 0)

tests.integrations.aws_lambda.test_aws test_init_error[python3.11]

Stack Traces | 15.9s run time

.../integrations/aws_lambda/test_aws.py:334: in test_init_error
    (event,) = envelope_items
E   ValueError: not enough values to unpack (expected 1, got 0)

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[szokeasaurusrex]

Looks like this maybe does not work on Python <3.12

[szokeasaurusrex]

Yeah, seems that %:z was introduced in 3.12, since it is not mentioned in the 3.11 docs