TestColorMaps.test_make|show_colormap fails with pytest

[olebole]

Bug report

Bug summary

When I use pytest for tests, the TestColorMaps.test_make_colormap and TestColorMaps.test_show_colormaps tests fail. Interestingly, this is not observed when running the tests with nosetest.

Code for reproduction

Run the tests with pytest

Actual outcome

=================================== FAILURES ===================================
_______________________ TestColorMaps.test_make_colormap _______________________

self = <unittest.case._Outcome object at 0x7fc84d03c730>
test_case = <yt.visualization.tests.test_color_maps.TestColorMaps testMethod=test_make_colormap>
isTest = True

    @contextlib.contextmanager
    def testPartExecutor(self, test_case, isTest=False):
        old_success = self.success
        self.success = True
        try:
>           yield

/usr/lib/python3.8/unittest/case.py:60: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/usr/lib/python3.8/unittest/case.py:676: in run
    self._callTestMethod(testMethod)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <yt.visualization.tests.test_color_maps.TestColorMaps testMethod=test_make_colormap>
method = <bound method requires_backend.<locals>.ffalse.<locals>.<lambda> of <yt.visualization.tests.test_color_maps.TestColorMaps testMethod=test_make_colormap>>

    def _callTestMethod(self, method):
>       method()
E       TypeError: <lambda>() takes 0 positional arguments but 1 was given

/usr/lib/python3.8/unittest/case.py:633: TypeError

Expected outcome

The tests succeed

Version Information

• Operating System: Debian unstable
• Python Version: 3.8
• yt version: 3.6.0
• Other Libraries (if applicable): pytest-4.6.9

[olebole]

Oops, when rechecking the error this seems to be a problem in unittest... Although I have no idea what this could be. Please re-open if this is caused by a yt bug.