TST: be more explicit about identifying qt4/qt5 imports

[tacaswell]

closes #14137

It is a bit heavy-handed, but seems better than relying on possibly varying import time collisions from upstreams.

[tacaswell]

force-pushed to fix indentation problem.

[QuLogic]

This is definitely helpful, though I can't quite figure out how to run one or the other. There doesn't seem to be a way in pytest to select a test based on a marker argument. And setting MPLBACKEND doesn't work since nothing 'locks it in' before the test setup switches backends (to Agg or Qt?Agg later).

Do you think we should add a secondary marker for backends as well?

[tacaswell]

pytest -k Qt4 and pytest -k Qt5 work for me to run one or the other.

(mpl_test) ✘  ~/src/p/matplotlib [fix_qt_tests ✭|⚑ 3]
sharon@17:14  ➤  pytest -k Qt5 -vs lib/matplotlib/tests/test_backend_qt.py 
======================================================================================================== test session starts =========================================================================================================
platform linux -- Python 3.6.8, pytest-4.4.1, py-1.8.0, pluggy-0.9.0 -- /tmp/mpl_test/bin/python3
cachedir: .pytest_cache
rootdir: /home/tcaswell/src/p/matplotlib, inifile: pytest.ini
collected 28 items / 16 deselected / 12 selected                                                                                                                                                                                     

lib/matplotlib/tests/test_backend_qt.py::test_fig_close[Qt5Agg] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-shift] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-lower] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-control] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-unicode_upper] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-unicode_lower] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-alt_control] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-control_alt] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-modifier_order] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-backspace] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-backspace_mod] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt5Agg-non_unicode_key] PASSED

============================================================================================== 12 passed, 16 deselected in 0.60 seconds ==============================================================================================
(mpl_test) ✔  ~/src/p/matplotlib [fix_qt_tests ✭|⚑ 3]
sharon@17:14  ➤  pytest -k Qt4 -vs lib/matplotlib/tests/test_backend_qt.py 
======================================================================================================== test session starts =========================================================================================================
platform linux -- Python 3.6.8, pytest-4.4.1, py-1.8.0, pluggy-0.9.0 -- /tmp/mpl_test/bin/python3
cachedir: .pytest_cache
rootdir: /home/tcaswell/src/p/matplotlib, inifile: pytest.ini
collected 28 items / 16 deselected / 12 selected                                                                                                                                                                                     

lib/matplotlib/tests/test_backend_qt.py::test_fig_close[Qt4Agg] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-shift] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-lower] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-control] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-unicode_upper] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-unicode_lower] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-alt_control] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-control_alt] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-modifier_order] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-backspace] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-backspace_mod] PASSED
lib/matplotlib/tests/test_backend_qt.py::test_correct_key[Qt4Agg-non_unicode_key] PASSED

============================================================================================== 12 passed, 16 deselected in 0.52 seconds ==============================================================================================
(mpl_test) ✔  ~/src/p/matplotlib [fix_qt_tests ✭|⚑ 3]

[QuLogic]

Unfortunately, that only works if the tests are parameterized (and thus have a backend name in the test name). There are additional Qt5-specific tests that weren't run in your example (which you can see from selected vs deselected counts).

[timhoffm]

I think, explicit backend selection for the tests is a different topic. But will leave open for now in case somebody feels the need to address it in this PR. Anybody can merge, if there's no further change wanted on the PR.