Datetime serialization and advice

[mpharrigan]

fixes #5301

See #5301 for motivation

[maffoo]

Minor comments, then LGTM

[maffoo]

Might also want to suggest that in cirq functions when we compare datetime instances (for equality or ordering) we should compare .timestamp() instead of comparing the datetime instances directly, since this will work for aware and naive datetimes just like serialization.

[mpharrigan]

wellllllllllllllll in #5301 (comment) (specifically the linked comment) the proposal was to compare the datetimes. This was in the context of dataclass-like member variables where the motivation was to mirror the behavior of the components. I suppose the tradeoffs could be different in functions where we just want it to work.

Propose adding:

Comparing (or testing equality) between "naive" and "aware" datetime objects throws
an exception.
If you are implementing a class that has datetime member variables, delegate equality
and comparison operators to the built-in datetime equality and comparison operators.
If you're writing a function that compares datetime objects, you can defensively promote
them to "aware" objects or use their .timestamp() properties for a comparison that will
never throw an exception.

[mpharrigan]

@maffoo does this sound good?

[maffoo]

SGTM