Bug: assert_produces_warning(None) not raising AssertionError with warning
- [x] closes #38620
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

These changes cause 23 test failures from `tm.assert_produces_warning(None)` now actually raising an `AssertionError` if a warning is given. What is the right approach here? Should those tests just be xfailed and an issue made to look into those warnings? wow

```
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-'UTC']
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-'US/Eastern']
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-'Asia/Tokyo']
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-'dateutil/US/Pacific']
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-'dateutil/Asia/Singapore']
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-'+01:15']
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-'-02:15']
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-'UTC+01:15']
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-'UTC-02:15']
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-tzutc()]
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-tzlocal()]
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-pytz.FixedOffset(300)]
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-<UTC>]
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-pytz.FixedOffset(-300)]
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-datetime.timezone.utc]
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-datetime.timezone(datetime.timedelta(seconds=3600))]
FAILED pandas/tests/arithmetic/test_timedelta64.py::TestTimedelta64ArithmeticUnsorted::test_tda_add_dt64_object_array[DataFrame-datetime.timezone(datetime.timedelta(days=-1, seconds=82800), 'foo')]
FAILED pandas/tests/frame/test_arithmetic.py::TestFrameArithmetic::test_binop_other[add-True-bool]
FAILED pandas/tests/frame/test_arithmetic.py::TestFrameArithmetic::test_binop_other[mul-True-bool]
FAILED pandas/tests/indexes/test_common.py::TestCommon::test_asi8_deprecation[int]
FAILED pandas/tests/indexes/test_common.py::TestCommon::test_asi8_deprecation[uint]
FAILED pandas/tests/indexes/test_common.py::TestCommon::test_asi8_deprecation[range]
FAILED pandas/tests/indexes/test_common.py::TestCommon::test_asi8_deprecation[repeats]

```

ok these at least should be straightforward to fix Bottom 4 failures can be fixed by removing `Int64Index` and `UInt64Index` from the tuple in https://github.com/pandas-dev/pandas/blob/ca52e3968058dce48c20070b3e9efc2ec42d7cf4/pandas/tests/indexes/test_common.py#L362-L363

Looks like this deprecation came from #37877, this removal would make sense if those indexes should give a `FutureWarning` with `.asi8` (as currently happens in master). cc @jbrockmendel  > Bottom 4 failures can be fixed by removing `Int64Index` and `UInt64Index` from the tuple in
> 
> https://github.com/pandas-dev/pandas/blob/ca52e3968058dce48c20070b3e9efc2ec42d7cf4/pandas/tests/indexes/test_common.py#L362-L363
> 
> Looks like this deprecation came from #37877, this removal would make sense if those indexes should give a `FutureWarning` with `.asi8` (as currently happens in master). cc @jbrockmendel

can also just xfail these 3 tests and open an issue as well. Green except Travis thanks @mzeitlin11 keep em coming!