PR for Pandas issue #14872 / fillna() TZ datetime64 rounded
Hi Jeff,

The change adds a "datetime64[ns, UTC]" object to the _DATELIKE_DTYPES set in pandas/types/common.py. 

No such object existed in this set and that is the reason the wrong method was executed on the inputs.

Object is created with "DatetimeTZDtype.__new__"

Thanks,
Rodolfo

 - [X] closes #14872
 - [ X] tests added / passed
 - [ X] passes ``git diff upstream/master | flake8 --diff`` (returns the same number of skips, errors and failures before changes)
 - [ ] whatsnew entry



this goes way overboard 

see the pull request which closed the original issue The changes in request #6587  added "dtype in _DATELIKE_DTYPES" to the method selection in the backfill_2d function (now in missing.py), 
```
-        _method = getattr(algos, 'backfill_2d_inplace_%s' % dtype, None)
-    elif is_datetime64_dtype(values):
+        _method = getattr(algos, 'backfill_2d_inplace_%s' % dtype.name, None)
+    elif dtype in _DATELIKE_DTYPES or is_datetime64_dtype(values):
```
I initially tried adding an extra is_datetime64tz_dtype() to this line with no success. After tracing the process with pdb I found that the values are already coerced to integers in pandas/core/internals.py,

```
    def _interpolate_with_fill(self, method='pad', axis=0, inplace=False,
                               limit=None, fill_value=None, coerce=False,
                               downcast=None, mgr=None):
        """ fillna but using the interpolate machinery """
        .
        .
        .
        values, _, fill_value, _ = self._try_coerce_args(values, fill_value)
```
before being passed to /pandas/core/missing.interpolate_2d and then to backfill_2d. The dtype information is passed as dtype=datetime64[ns, UTC]. When it gets to these lines in the backfill_2d function,
```
elif dtype in _DATELIKE_DTYPES or is_datetime64_dtype(values):
      method = _backfill_2d_datetime
```
the return is always false because datetime64[ns, UTC] is not in _DATELIKE_DTYPES and the 'values' are integers.

Another way would be to either replace datetime64[ns, UTC] with datetime64[ns] before we get to this line or skip the coercion somehow. Unless I am missing something.
 so this should simply be changed to pass the actual value (and not coerced ones) when this is called and drop al of the _DATELIKE_DTYPES that is really old code

use the is_datetime64_dtype and is_datetime64tz_dtype 

 actually you just need to check the dtype instead (of the values)

e.g.
```
(Pdb) p is_datetime64tz_dtype(dtype)
True
```
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14905?src=pr) is 84.57% (diff: 75.00%)
> Merging [#14905](https://codecov.io/gh/pandas-dev/pandas/pull/14905?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **0.01%**

```diff
@@             master     #14905   diff @@
==========================================
  Files           144        144          
  Lines         51043      51058    +15   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43160      43181    +21   
+ Misses         7883       7877     -6   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [73bc6cf...18802b4](https://codecov.io/gh/pandas-dev/pandas/compare/73bc6cfb35a886b18955b1f0a07f7647f1f1d409...18802b4c96705ca0df78518f485d3975f1615387?src=pr) Done! I was indeed missing something. Thank you for the feedback! thanks!