docs: install package in ReadTheDocs build (#3032)
docs: install package in ReadTheDocs build (#3032)

Need to install ourselves so that we can introspect on version
using `setuptools_scm` in `docs/conf.py`.

Signed-off-by: Milas Bowman <bf766bbdae72109c448c7f902860a03abdcfef7b@docker.com>
docker/docker-py,docker/docker-py