[ENH] Accept UTC offsets in datetimes

[effigies]

As discussed in #1968, non-UTC timezones can be useful for coordinating acquisitions across time zones. The spec currently adopts RFC3339 with the exception that nonzero UTC offsets are not permitted, and the Z offset is optional to allow indicating local time.

For backwards compatibility, I propose allowing the offsets to remain optional, but permitting the full range of RFC3339 offsets.

Separately, the current regular expression does not express the current rule, so I will open another PR against the maintenance branch for that.

[nellh]

Verified the structure of the regular expression and this looks good to me.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.58%. Comparing base (3b6d2f7) to head (2fc3090).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2001   +/-   ##
=======================================
  Coverage   82.58%   82.58%           
=======================================
  Files          17       17           
  Lines        1499     1499           
=======================================
  Hits         1238     1238           
  Misses        261      261           

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