BUG/DEPR: combine dtype fixes
- [x] closes #7630, closes #10567 (fix `combine_first` dtype)
- [x] closes #13947 (`_find_common_type` to support extension types)
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry
- `_find_common_type` also needs to handle numpy datetimelike to be compat with pandas because:

```
np.find_common_type([np.dtype('datetime64[ns]'), np.dtype('datetime64[ns]')], [])
# dtype('O')
```
- another `comibine_first` related issue  #7509 is NOT fixed because it is related to filling with `align`

xref to #4094 (though that's another story)
 very minor change. lgtm otherwise. ping when green.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13970?src=pr) is 85.28% (diff: 100%)

> Merging [#13970](https://codecov.io/gh/pydata/pandas/pull/13970?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.01%**

``` diff
@@             master     #13970   diff @@
==========================================
  Files           139        139          
  Lines         50216      50206    -10   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42817      42818     +1   
+ Misses         7399       7388    -11   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [70f361b...2046cb5](https://codecov.io/gh/pydata/pandas/compare/70f361ba78fd9385da469e5a13cca6e9685c8b1a...2046cb57a5871c618829e1772f35bc63147d8d13?src=pr)
 Not really sure we should actually deprecate `DataFrame.combine`. Although I have never used it, it does serve a purpose that is not possible to achieve with "the flexible arithmetic methods like `add`" when you pass it a custom function (unlike the deprecated `combineAdd`, which definitely can be done with `add`)
 I've commented on the original (#10734). 
 ok, so as discussed in the issue. let's take out the combine deprecation from this PR. ping when ready.
 Exclude `combine` deprecation, and now all green.
 thanks!
