tests: address mutable class vars and naive datetime in Python code

tests: address mutable class vars and naive datetime in Python code

Mark Python mutable class variables with ClassVar, to denote that the
danger this can cause has been considered.  Since any change made to
these in any object affects all other objects, this can cause locality
errors. However, as used in the test suite, they are are never modified
and so they are annotated as being intended.

Always set a timezone in datetime objects, as mixing naive and
timezone-aware object can cause errors.

These fix ruff rules DTZ005, RUF012.