The import path mangling is not relevant (anymore?) for the two
ways of running tests:
* `python3 test/testall.py` (see CONTRIBUTING.rst):
The `testall.py` script already adds the project path to `sys.path`.
* `tox -e py-cov`: this command is supposed to be run from the project
path. Thus, the current directory is already the first of location
in `sys.path`.
The previous mangling of the import path while loading a module could
lead to unwanted side-effects hidden in an unexpected location.
Instead, import path mangling should take place in the script being
called by the user (here: `testall.py`).