Bugfix/graypy polluting site packages

[nklapste]

Resolves https://github.com/severb/graypy/issues/131

Fixes issue where installing graypy would also copy the project's tests directory to the Python environment's site-packages.

[finswimmer]

Hey,

unfortunately there was no process in this project for a long time. The issue this PR is trying to resolve is very annoying. The fix is quite easy. So it would be very helpful, if there would be at least a release that contains that fix. :pray:

fin swimmer

[skewty]

LGTM. Do we need to fork this project because it is dead?