Switch from freezegun to time-machine.

[adamchainz]

Problem

freezegun is slow.

Solution

time-machine is more accurate and faster.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• [n/a] Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• [n/a] Pay attention to backward compatibility, or if it breaks it, explain why.
• [n/a] Update documentation (if relevant).

[foarsitter]

Thanks @adamchainz

[foarsitter]

@adamchainz I will have to revert this PR since it breaks the tests. Do you have any clue what's going wrong?

[foarsitter]

Setting USE_TZ = True solves the issue.