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

Merging #441 (938e566) into main (9383257) will decrease coverage by 49.16%.
The diff coverage is 31.81%.

@@             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	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cf_xarray/helpers.py	25.64% <ø> (-40.18%)	⬇️
cf_xarray/tests/test_accessor.py	29.06% <31.81%> (-64.86%)	⬇️

... and 19 files with indirect coverage changes