pytest import from root tests folder

[brunolnetto]

From time to time a guinea pig appears to cheer its headache complains with war-boars like you. Today, it is myself.

I wrote an euler-diagram library, very useful for data analysis on sets. So far, so good. After many prior configurations like tox and GitHub actions workflow for test coverage, I still find a problem running tests on my gh-actions CI. Please, be kind to me, I am tired to try fix this workflow. :-/