Clean up documentation of dependencies

We would previously document dependencies which are already expressed
programmatically in `pyproject.toml`, so this added information not
useful for readers, but also introduced potential for things to get out
of sync (which they had already). Just drop anything already expressed
in the project config.

Author: bbannier

doc/quickstart.rst

@@ -7,11 +7,7 @@ Quickstart Guide
 Dependencies
 ------------
 
-* Python 3.6+
 * git: https://git-scm.com
-* GitPython: https://pypi.python.org/pypi/GitPython
-* semantic_version: https://pypi.python.org/pypi/semantic_version
-* btest: https://pypi.python.org/pypi/btest
 
 Note that following the :program:`zkg` `Installation`_ process via
 :program:`pip3` will automatically install its dependencies for you.