COMPAT: IntegerXXDtypes string repr cause numpy deprecation warnings
The 'numeric-style' type codes are deprecated in numpy, but they clash with our pandas integer dtypes strings (with the capital):

```
In [7]: np.dtype('Int64')                                     
/home/joris/miniconda3/envs/dev/bin/ipython:1: DeprecationWarning: Numeric-style type codes are deprecated and will result in an error in the future.
  #!/home/joris/miniconda3/envs/dev/bin/python
Out[7]: dtype('int64')
```

This causes some deprecation warnings in `pd.core.dtypes.common._get_dtype_type`, which surfaces those through eg `is_integer_dtype`:
```
In [1]: import warnings                                      

In [2]: warnings.simplefilter('always', DeprecationWarning)

In [3]: pd.api.types.pandas_dtype('Int64')                                                
Out[3]: <pandas.core.arrays.integer.Int64Dtype at 0x7fa8dc28c518>

In [4]: pd.core.dtypes.common._get_dtype_type('Int64')                                             
/home/joris/scipy/pandas/pandas/core/dtypes/common.py:1864: DeprecationWarning: Numeric-style type codes are deprecated and will result in an error in the future.
  return _get_dtype_type(np.dtype(arr_or_dtype))
Out[4]: numpy.int64

In [6]: pd.api.types.is_integer_dtype('Int64')                         
/home/joris/scipy/pandas/pandas/core/dtypes/common.py:1864: DeprecationWarning: Numeric-style type codes are deprecated and will result in an error in the future.
  return _get_dtype_type(np.dtype(arr_or_dtype))
Out[6]: True
```