Gaclac reader tests fail when pygac is installed and is run with other tests

[mraspaud]

I have no clue why, but with this PR, these import statements all return in mocks, causing a test failure:

satpy/satpy/tests/reader_tests/test_avhrr_l1b_gaclac.py

Lines 140 to 147 in d556c80

	def test_init(self):
	"""Test GACLACFile initialization."""
	from pygac.gac_klm import GACKLMReader
	from pygac.gac_pod import GACPODReader
	from pygac.lac_klm import LACKLMReader
	from pygac.lac_pod import LACPODReader

Any clue why that would happen with my PR, which is just doing stuff with warning filters and loggers?

These failures happen on my local machine if and only if local pygac is installed, and I run the new tests:

pytest test_utils.py reader_tests/test_avhrr_l1b_gaclac.py (or a generic pytest) while pygac is installed gives me:

================================================================================================================== short test summary info ===================================================================================================================
FAILED reader_tests/test_avhrr_l1b_gaclac.py::TestGACLACFile::test__slice - ValueError: Given start line 5 exceeds scanline range 2                                                                                                                          
FAILED reader_tests/test_avhrr_l1b_gaclac.py::TestGACLACFile::test_init - AssertionError: <class 'pygac.gac_pod.GACPODReader'> is not <MagicMock name='mock.gac_pod.GACPODReader' id='140567597024208'> : Wrong reader class assigned to NSS.GHRR.NA.D79184...
FAILED reader_tests/test_avhrr_l1b_gaclac.py::TestGACLACFile::test_strip_invalid_lat - TypeError: ufunc 'isnan' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''  
========================================================================================================= 3 failed, 21 passed, 41 warnings in 6.24s ==========================================================================================================

pytest reader_test/test_avhrr_l1b_gaclac.py — all tests pass.

Originally posted by @gerritholl in #1554 (comment)