Add 'expired' method to Signed class

Description of issue or feature request: Add a method of Signed metadata class returning information about metadata expiration given a reference time.

Current behavior: Signed class already contains expires, a metadata expiration datetime object.

Expected behavior: A method exists that checks the ‘expires’ attribute against a reference time. def is_expired(self, reference_time: float) -> bool

[sechkova]

No strong preferences, the client will use whatever we choose 😃 I agree with using datetime objects plus the datetime module seems to be less platform dependent than time (which can be used to return UNIX time).

Let’s make sure we enforce UTC times everywhere.

[joshuagl]

Fully agreed with both comments. I have a WIP patch that takes a datetime object for reference_time and defaults to datetime.now() when no reference_time is passed.

I believe the motivating use-case is expiration checks in the experimental client, so I’ll wait for @sechkova to comment on whether a datetime object is useful.