Default installation fails pytest

Several tests fail with the following error:

Can't find model 'en_core_web_sm'. It doesn't seem to be a Python package or a valid path to a data directory.

This is a clean virtual environment based on Python 3.9 following the "From Source" instructions. 113 tests pass (5 are skipped) after running python -m spacy download en_core_web_sm.

I will send a PR to clarify the documentation.

Thanks! spaCy models are sometimes challenging to install automatically. A clarification to the documentation would be great.

Closing for now, please reopen if the documentation is still not clear enough