COMPAT: extension dtypes (DatetimeTZ, Categorical) are now Singleton cached objects

[jreback]

allows for proper is / == comparisons
Had this odd semantic difference as these were really different objects (though they DID hash the same)
This doesn't actually affect any user code.

In [1]: from pandas.core import common as com

In [2]: t1 = com.DatetimeTZDtype('datetime64[ns, US/Eastern]')

In [3]: t2 = com.DatetimeTZDtype('datetime64[ns, US/Eastern]')

In [4]: t1 == t2
Out[4]: True

In [5]: t1 is t2
Out[5]: False

In [6]: hash(t1)
Out[6]: 5756291921003024619

In [7]: hash(t2)
Out[7]: 5756291921003024619

[codecov-io]

Current coverage is 84.20%

Merging #13285 into master will increase coverage by <.01%

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+ Hits          42592      42613    +21   
+ Misses         7999       7994     -5   
  Partials          0          0          

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