ENH - When using the copybutton in code blocks, exclude copying the prompt (#2036)

When I use the copybutton in code blocks, I expect to be able to paste
the code into my terminal without having to delete the prompt that gets
copied. This PR fixes that issue by excluding CSS classes that Pygments
generates in code blocks.

See
https://sphinx-copybutton.readthedocs.io/en/latest/use.html#automatic-exclusion-of-prompts-from-the-copies

Author: stevepiercy

docs/conf.py

@@ -113,6 +113,7 @@
 # Exclude copy button from appearing over notebook cell numbers by using :not()
 # The default copybutton selector is `div.highlight pre`
 # https://github.com/executablebooks/sphinx-copybutton/blob/master/sphinx_copybutton/__init__.py#L82
+copybutton_exclude = ".linenos, .gp"
 copybutton_selector = ":not(.prompt) > div.highlight pre"
 
 # -- Options for HTML output -------------------------------------------------