Matplotlib from conda missing baseline images when `pyinseq --test` is called

[eburgoswisc]

The --test option of pyinseq, which uses pytest in the rootdir, throws this exception when called:

$ pyinseq --test
2021-08-09 14:55 - INFO - pyinseq - Process command line arguments
=============================================================================================== test session starts ===============================================================================================
platform darwin -- Python 3.6.13, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
rootdir: /Users/emanuelburgos/opt/miniconda3/envs/test_pyinseq_py36/lib/python3.6/site-packages
plugins: cov-2.12.1
collected 0 items / 1 error                                                                                                                                                                                       

===================================================================================================== ERRORS ======================================================================================================
__________________________________________________________________________________________ ERROR collecting test session __________________________________________________________________________________________
../importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
<frozen importlib._bootstrap>:994: in _gcd_import
    ???
<frozen importlib._bootstrap>:971: in _find_and_load
    ???
<frozen importlib._bootstrap>:941: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:219: in _call_with_frames_removed
    ???
<frozen importlib._bootstrap>:994: in _gcd_import
    ???
<frozen importlib._bootstrap>:971: in _find_and_load
    ???
<frozen importlib._bootstrap>:955: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:665: in _load_unlocked
    ???
<frozen importlib._bootstrap_external>:678: in exec_module
    ???
<frozen importlib._bootstrap>:219: in _call_with_frames_removed
    ???
matplotlib/tests/__init__.py:7: in <module>
    'The baseline image directory does not exist. '
E   OSError: The baseline image directory does not exist. This is most likely because the test data is not installed. You may need to install matplotlib from source to get the test data.
============================================================================================= short test summary info =============================================================================================
ERROR  - OSError: The baseline image directory does not exist. This is most likely because the test data is not installed. You may need to install matplotlib from source to get the test data.

I believe this happens because calling pytest.main() does not correctly append the test directories to the PYTHONPATH. For now, you can clone the source code (or cd into site-packages/pyinseq in your python architecture) to verify the software. However, this will not cause issues since TravisCI testing frameworks are implemented in the pyinseq github

[mandel01]

The tests passed on the TravisCI instance. It looks like that is because the software is installed via pip there. Once this issue is addressed, can we redesign the Travis script so that the installation on Travis mirrors a user's conda installation?

It seems to me there is a chicken-and-egg issue here (the commits here need to be in place before they appear in conda!). Perhaps there is a clever solution...