update: uses UTC for timestamps

[namsnath]

Summary of change

• Makes timestamps consistent, uses UTC timezone

Related issues

• tests/test_session.py::test_token_cookie_expires failed #554

Test Plan

• Run supertokens-python test suite

Documentation changes

(If relevant, please create a PR in our docs repo, or create a checklist here highlighting the necessary changes)

Checklist for important updates

• Changelog has been updated
• coreDriverInterfaceSupported.json file has been updated (if needed)
  • Along with the associated array in supertokens_python/constants.py
• frontendDriverInterfaceSupported.json file has been updated (if needed)
• Changes to the version if needed
  • In setup.py
  • In supertokens_python/constants.py
• Had installed and ran the pre-commit hook
• Issue this PR against the latest non released version branch.
  • To know which one it is, run find the latest released tag (git tag) in the format vX.Y.Z, and then find the latest branch (git branch --all) whose X.Y is greater than the latest released tag.
  • If no such branch exists, then create one from the latest released branch.
• If have added a new web framework, update the supertokens_python/utils.py file to include that in the FRAMEWORKS variable
• If added a new recipe that has a User type with extra info, then be sure to change the User type in supertokens_python/types.py
• Make sure that syncio / asyncio functions are consistent.
• If access token structure has changed
  • Modified test in tests/sessions/test_access_token_version.py to account for any new claims that are optional or omitted by the core

Remaining TODOs for this PR