datetime.utcnow() deprecated in Python 3.12

[petercoles]

No solution proposed at this point (as it would need to be explicitly tested against older versions of Python).

This is just a marker that users will start to run into problems with Flumine's use of naive datetime objects at some time in an as yet unknown future version of Python.

[mzaja]

datetime.datetime.utcnow() calls need to be replaced with datetime.datetime.now(datetime.timezone.utc). This is backwards compatible with Python 3.8, the earliest supported version: https://docs.python.org/3.8/library/datetime.html#datetime.datetime.utcnow