copybutton_exclude: Add .gp to default exclusion

- `.linenos` is the line numbers, and `.gp` is the tag from the
  pygments highlighter for things like the `$` prompt.  Upstream
  sphinx CSS excludes `.gp` from being copied, see the analysis here:

  executablebooks#138 (comment)

- If upstream sphinx excludes `.gp`, we probably should as well.

sphinx_copybutton/__init__.py

@@ -81,7 +81,7 @@ def setup(app):
     app.add_config_value("copybutton_here_doc_delimiter", "", "html")
     app.add_config_value("copybutton_image_svg", "", "html")
     app.add_config_value("copybutton_selector", "div.highlight pre", "html")
-    app.add_config_value("copybutton_exclude", ".linenos", "html")
+    app.add_config_value("copybutton_exclude", ".linenos, .gp", "html")
 
     # DEPRECATE THIS AFTER THE NEXT RELEASE
     app.add_config_value("copybutton_image_path", "", "html")