Make copybutton messages translatable

[ccwang002]

This PR implements #23 which make the messages of copybutton (>>>) translatable.

When I tested my change using the master branch of Python (branch 3.7 doesn't seem to use python-docs-theme), I couldn't get it read my modified layout.html. However, if I manually introduced the change, the pot files would include the new strings and the translation worked.

I think it is better to have someone more familiar with the project structure of PEP 545 to test my implementation before it is merged.

[JulienPalard]

There's a little issue, the command gathering translations strings:

sphinx-build -Q -b gettext -D gettext_compact=0 Doc pot/

won't pick it up, so translation teams won't see it as a sentence to translate. I don't have the time right now to check if there's a simple way to solve this, maybe Sphinx has a way to look for translations strings in configured theme?

[theacodes]

These resources might be helpful:

1. Add localization language support readthedocs/sphinx_rtd_theme#403
2. Added localization language support readthedocs/sphinx_rtd_theme#405

[septatrix]

I think I looked into this in the past and it was possible to directly translate the strings in the javascript file