docs: install package in ReadTheDocs build

[milas]

The setuptools_scm docs have a recommended way to query
the version for Sphinx integration which we use:

docker-py/docs/conf.py

Lines 62 to 70 in e901eac

	# The version info for the project you're documenting, acts as replacement for
	# |version| and |release|, also used in various other places throughout the
	# built documents.
	#
	# see https://github.com/pypa/setuptools_scm#usage-from-sphinx
	from importlib.metadata import version
	release = version('docker')
	# for example take major/minor
	version = '.'.join(release.split('.')[:2])

However, this is currently failing because it requires the docker
package (this one!) to be importable, so it really needs to have
the equivalent of pip install -e . done on it.

This updates the RTD config to actually install this package as
part of the build vs just its dependencies, so that the version
introspection (hopefully) works.