Use the "sphinx-copybutton" extension in documentation

[rahulporuri]

This PR does the following:

• Use the sphinx_copybutton extension when building the Sphinx documentation
• Make sphinx-copybutton a docs extra for the chaco package
• Add a pypi_dependencies section to the ci/edmtool.py utility and add sphinx-copybutton as a PyPI dependency for the moment.

Ref https://sphinx-copybutton.readthedocs.io/en/latest/