pin sphinx-copybutton and change config

pin sphinx-copybutton and change config

sphinx-copybutton introduced a new feature
in 0.5.1 which includes a default configuration
that breaks the regexp prompt matching scheme.

set copybutton_exclude to not include ".gp" as that's the class
where we exactly look for the prompts we are matching.

pin sphinx-copybutton at 0.5.1 to avoid future problems.

Change-Id: Ie03bc27a9190e71b63fc68b484f23e53b8cb72dc
References: https://github.com/executablebooks/sphinx-copybutton/issues/185