Disable usedevelop for some tox testenvs

Tox's usedevelop option implicitly installs the project and its
dependencies, and is enabled in the base testenv. Disable it in
testenvs which don't make use of the installed project, but add the
project dependencies to the docs testenv since it needs them to be
able to generate the API docs. This results in a fairly substantial
speedup of >10% for a full `tox` invocation.