BUG: tm.assert_produces_warning() doesn't work with expected_warning=None or False

[mzeitlin11]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Code Sample, a copy-pastable example

import warnings
import pandas._testing as tm

with tm.assert_produces_warning(None):
    warnings.warn(UserWarning)

with tm.assert_produces_warning(False):
    warnings.warn(RuntimeWarning)

Problem description

Docs for this function say "To check that no warning is returned, specify False or None". It also includes the second example as something that should raise an AssertionError. However, both these examples do not raise.

Expected Output

Raise AssertionError for both.

Output of pd.show_versions()

INSTALLED VERSIONS

commit : 9a46a4b
python : 3.8.6.final.0
python-bits : 64
OS : Darwin
OS-release : 19.6.0
Version : Darwin Kernel Version 19.6.0: Thu Oct 29 22:56:45 PDT 2020; root:xnu-6153.141.2.2~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 1.3.0.dev0+125.g9a46a4b70
numpy : 1.19.4
pytz : 2020.4
dateutil : 2.8.1
pip : 20.3.1
setuptools : 49.6.0.post20201009
Cython : 0.29.21
pytest : 6.2.0
hypothesis : 5.43.3
sphinx : 3.3.1
blosc : None
feather : None
xlsxwriter : 1.3.7
lxml.etree : 4.6.2
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.19.0
pandas_datareader: None
bs4 : 4.9.3
bottleneck : 1.3.2
fsspec : 0.8.4
fastparquet : 0.4.1
gcsfs : 0.7.1
matplotlib : 3.3.3
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.5
pandas_gbq : None
pyarrow : 2.0.0
pyxlsb : None
s3fs : 0.4.2
scipy : 1.5.3
sqlalchemy : 1.3.20
tables : 3.6.1
tabulate : 0.8.7
xarray : 0.16.2
xlrd : 1.2.0
xlwt : 1.3.0
numba : 0.52.0

[mzeitlin11]

take

[simonjayhawkins]

first bad commit: [77811c7] ENH: implement matching warning message (#37263)