Freeze numpy version

[Bachibouzouk]

Addresses temporarily #908 by treating symptoms but not the cause

Changes proposed in this pull request:

• Set numpy version to lower or equal than 1.19.4

The following steps were realized, as well (if applies):

• [x] Use in-line comments to explain your code
• [ ] Write docstrings to your code (example docstring)
• [ ] For new functionalities: Explain in readthedocs
• [ ] Write test(s) for your new patch of code (pytests, assertion debug messages)
• [x] Update the CHANGELOG.md
• [ ] Apply black (black . --exclude docs/)
• [ ] Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

_{For more information on how to contribute check the CONTRIBUTING.md.}