Fix _can_hold_element for datetimelike blocks
I've got a bunch of branches going that touch this, easier to just get it right once.
> so this basically unified DatetimeBlock and DateimtTZBlock for _can_hold_element?

Yes.  ATM DatetimeTZBlock._can_hold_element is ExtensionBlock._can_hold_element, which always returns True lgtm. ping on green. The CI fail is weird.  It’s getting a warning about elementwise comparison but it’s a scalar being compared. ping gentle pint; this is a semi-blocker > will try this out in a dedicated branch
 
Thanks. +1 on the changes here, pending the resolution of `is_integer` / `is_integer_strict` (can we think of a different suffix than `_strict`? That doesn't really convey that (just) time delta are excluded).  > Does the whole world break if you just add the exclusion of timedelta64 to is_integer

Nope, just opened #27401 

I think the thing to do here is to revert the last commit to get rid of is_integer_strict and move forward with the more verbose variant, then trim that if/when #27401 goes in. ping thanks @jbrockmendel 