Corrections from @jreback comments

* Added test for is_timedelta64_dtype
* Added double backticks in what's new

Author: Jean-Mathieu Deschenes

Diff for: doc/source/whatsnew/v0.20.3.txt

@@ -36,7 +36,7 @@ Performance Improvements
 
 Bug Fixes
 ~~~~~~~~~
-- Bug in a DataFrame/Series with a TimedeltaIndex when slice indexing (:issue:`16637`)
+- Bug in a DataFrame/Series with a ``TimedeltaIndex`` when slice indexing (:issue:`16637`)
 - Fixed issue with dataframe scatter plot for categorical data that reports incorrect column key not found when categorical data is used for plotting (:issue:`16199`)
 - Fixed a pytest marker failing downstream packages' tests suites (:issue:`16680`)
 

Diff for: pandas/tests/dtypes/test_common.py

@@ -200,7 +200,7 @@ def test_is_datetime64tz_dtype():
 def test_is_timedelta64_dtype():
     assert not com.is_timedelta64_dtype(object)
     assert not com.is_timedelta64_dtype([1, 2, 3])
-
+    assert not com.is_timedelta64_dtype(np.array([], dtype=np.datetime64))
     assert com.is_timedelta64_dtype(np.timedelta64)
     assert com.is_timedelta64_dtype(pd.Series([], dtype="timedelta64[ns]"))