namespace consistency for test_dtype.py (#37843)
namespace consistency for test_dtype.py (#37843)

* namespace consistency for test_dtype.py

* namespace inconsistency fix to test_interval.py

* reverting changes to test_dtype.py

* fix flake8 error

* fix Interval change

* adding back pd.IntervalDtype
pandas-dev/pandas,rs2/pandas,pandas-dev/pandas,pandas-dev/pandas,jreback/pandas,jorisvandenbossche/pandas,dsm054/pandas,jorisvandenbossche/pandas,datapythonista/pandas,rs2/pandas,datapythonista/pandas,gfyoung/pandas,jreback/pandas,rs2/pandas,jreback/pandas,jreback/pandas,datapythonista/pandas,dsm054/pandas,dsm054/pandas,gfyoung/pandas,dsm054/pandas,gfyoung/pandas,gfyoung/pandas,rs2/pandas,pandas-dev/pandas,dsm054/pandas,jorisvandenbossche/pandas,datapythonista/pandas,jreback/pandas,gfyoung/pandas,jorisvandenbossche/pandas