Remove usages of deprecated datetime.utcnow() and datetime.utcfromtimestamp()

[kozlek]

Changes:

• removed usages of datetime.utcnow() and datetime.utcfromtimestamp()
• added freezegun as test dependency to handle properly time patching during tests

Notes:

• the chosen approach in this PR uses aware datetimes by default and downgrade to naive datetime if needed. Before it was the contrary.
• aware_utcnow and datetime_from_epoch can still return naive datetime if settings.USE_TZ is False (original behaviour) -> we might want to force aware datetime in a future release
• make_utc function has been kept but it is only used by our tests, not library code

[Andrew-Chen-Wang]

Thanks for this!

[50-Course]

...aware_utcnow and datetime_from_epoch can still return naive datetime if settings.USE_TZ is False (original behaviour) -> we might want to force aware datetime in a future release

Thank you for this PR! Would be approving changes since, there is no breakage in library code.