fix: Accepting time-zone aware expiration datetime only to prevent hard-to debug issues with invalid signatures

[pawelkedzior-gl]

Description

Fixes #12692

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• [yes] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [yes] README is updated to include all relevant information
• [kind of] Tests pass: nox -s py-3.9 (see Test Environment Setup)

pytest
(...)
snippets_test.py::test_sign_url PASSED                                                                                                                                                                                                                                            [ 16%]
snippets_test.py::test_sign_url_raise_exception_on_naive_expiration_datetime PASSED                                                                                                                                                                                               [ 33%]
snippets_test.py::test_sign_url_prefix PASSED                                                                                                                                                                                                                                     [ 50%]
snippets_test.py::test_sign_url_prefix_raise_exception_on_naive_expiration_datetime PASSED                                                                                                                                                                                        [ 66%]
snippets_test.py::test_sign_cookie PASSED                                                                                                                                                                                                                                         [ 83%]
snippets_test.py::test_sign_cookie_raise_exception_on_naive_expiration_datetime PASSED

• [yes] Lint pass: nox -s lint (see Test Environment Setup)

cp ../noxfile-template.py noxfile.py
nox -s lint
nox > Running session lint
nox > Creating virtual environment (virtualenv) using python3 in .nox/lint
nox > python -m pip install flake8 flake8-import-order
nox > flake8 --show-source --builtin=gettext --max-complexity=20 --import-order-style=google '--exclude=.nox,.cache,env,lib,generated_pb2,*_pb2.py,*_pb2_grpc.py' --ignore=ANN101,ANN102,E121,E123,E126,E203,E226,E24,E266,E501,E704,W503,W504,I202 --max-line-length=88 --application-import-names .nox,env,noxfile,snippets,snippets_test .
nox > Session lint was successful.

• [no] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [no] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [no] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [no] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [yes] Please merge this PR for me once it is approved

[glasnt]

Pending CI, this looks correct. Will approve once Kokoro agrees.