Add handling of cftime bounds

[znichollscr]

At the moment this just adds a failing test that shows that add_bounds fails if the times are cftimes. I'm not sure what the best solution would be.

[znichollscr]

@mathause any thoughts on this one? (Calling on a familiar name :))

[codecov]

Codecov Report

❌ Patch coverage is 31.81818% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.24%. Comparing base (9383257) to head (938e566).
⚠️ Report is 119 commits behind head on main.

Files with missing lines	Patch %	Lines
cf_xarray/tests/test_accessor.py	31.81%	10 Missing and 5 partials ⚠️

❗ There is a different number of reports uploaded between BASE (9383257) and HEAD (938e566). Click for more details.

HEAD has 18 uploads less than BASE

Flag	BASE (9383257)	HEAD (938e566)
unittests	12	0
mypy	8	2

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #441       +/-   ##
===========================================
- Coverage   86.40%   37.24%   -49.16%     
===========================================
  Files          22       21        -1     
  Lines        3994     3936       -58     
  Branches      194      199        +5     
===========================================
- Hits         3451     1466     -1985     
- Misses        491     2271     +1780     
- Partials       52      199      +147     

Flag	Coverage Δ
mypy	37.24% <31.81%> (-0.04%)	⬇️
unittests	?

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