Fixing pandas/tests/indexes/datetimes/test_ops.py to utilize python3 …
Updated pandas/tests/indexes/datetimes/test_ops.py to utilize python3 format strings.  Ran the test and it seems good.  I moved the msg into the for loop because it seems cleaner with the new format string logic. 
ref [#29886](https://github.com/pandas-dev/pandas/issues/29886) and [#29547](https://github.com/pandas-dev/pandas/issues/29547).