Fix string slice indexing for a length-1 CFTimeIndex

[spencerkclark]

• Closes CFTimeIndex improperly handles string slice for length-1 indexes #2165
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including whats-new.rst for all changes and api.rst for new API (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

The issue is that both is_monotonic_decreasing and is_monotonic_increasing return True for a length-1 index; therefore an additional check is needed to make sure the length of the index is greater than 1 in CFTimeIndex._maybe_cast_slice_bound. This is similar to how things are done in DatetimeIndex._maybe_cast_slice_bound in pandas.

[shoyer]

thanks!