Vendored libraries

sphinxcontrib-domaintools

This was vendored in to fix warning that occur when building cocotb's documentation. It could not be fixed upstream because the project has been abandoned.

Based on sphinx-contrib/domaintools@cc6a5dcb3204325f52d160a0da7e097f3a825c8c. Applied sphinx-contrib/domaintools#7 and sphinx-contrib-domaintools#13.