Can't run unit tests locally on MacOS

🐛 Bug

It is not possible to run unit tests locally on MacOS X (10.12).

To Reproduce

Steps to reproduce the behavior:

Create a conda environment and install all of the packages as suggested here.
Run the following code: pytest tests

The unit tests fail and generate an error:

ImportError while importing test module '/Users/melekhi1/projects/solt/tests/test_data_core.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
tests/test_data_core.py:4: in <module>
import cv2
../../anaconda2/envs/solt_test_env/lib/python3.8/site-packages/cv2/__init__.py:3: in <module>
from .cv2 import *
E   ImportError: dlopen(/Users/melekhi1/anaconda2/envs/solt_test_env/lib/python3.8/site-packages/cv2/cv2.cpython-38-darwin.so, 2): Symbol not found: _inflateValidate
E     Referenced from: /Users/melekhi1/anaconda2/envs/solt_test_env/lib/python3.8/site-packages/cv2/.dylibs/libpng16.16.dylib (which was built for Mac OS X 10.13)
E     Expected in: /usr/lib/libz.1.dylib
E    in /Users/melekhi1/anaconda2/envs/solt_test_env/lib/python3.8/site-packages/cv2/.dylibs/libpng16.16.dylib

Expected behavior

The unit tests should pass


=============================== test session starts ================================
platform darwin -- Python 3.8.2, pytest-3.6.4, py-1.8.1, pluggy-0.7.1
rootdir: /Users/melekhi1/projects/solt, inifile:
plugins: pep8-1.0.6, flake8-1.0.2, cov-2.6.0
collected 701 items

tests/test_base_transforms.py .............................................. [ 6%] ............................................................................ [ 17%] ..... [ 18%] tests/test_data_core.py .................................................... [ 25%] ............................................................................ [ 36%] ............................................................................ [ 47%] .................................................................. [ 56%] tests/test_transforms.py ................................................... [ 63%] ............................................................................ [ 74%] ............................................................................ [ 85%] ............................................................................ [ 96%] ......... [ 97%] tests/test_utils.py ................ [100%]

============================ 701 passed in 6.79 seconds ============================


## **Suggestions**
It seems the bug is related to the latest version of `opencv-python-headless` (4.2.0.32) package. Apparently, its previous version, **4.1.2.30**, could solve the issue at least for this particular platform (MacOS 10.12). It would be great to test this solution on more recent MacOS.

Oulu-IMEDS / solt