datetime.utcnow() is deprecated: stop using it

[jku]

utcnow() reference docs:

Deprecated since version 3.12: Use datetime.now() with UTC instead.

We have 23 uses of utcnow() (mostly tests and examples): all seem straight forward to change to now() with utc argument

[Mansur908]

Hello @jku. Could you please assign this issue to me? Thank you.