DEPR: DataFrame.get_dtype_counts
- [x] xref #18262
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Unfortunately, `.dtype.value_counts()` isn't a perfect replacement for `get_value_counts()` (IMO it's better), so the test replacements calls the internal representation:

```
In [2]: df = pd.DataFrame([[1, 1.5, pd.Timestamp.now(), True]])

In [3]: df
Out[3]:
   0    1                          2     3
0  1  1.5 2019-06-30 11:55:24.380126  True

In [7]: df.get_dtype_counts().index
Out[7]: Index(['float64', 'int64', 'datetime64[ns]', 'bool'], dtype='object')

In [8]: df.dtypes.value_counts().index
Out[8]: Index([datetime64[ns], bool, float64, int64], dtype='object')
```