Update conf.py

The docs will successfully build and be accessible from https://developer.skatelescope.org/en/latest/projects/list.html once this MR is merged. Currently the build from master fails.

There is no need to import tango or skabase when building the Readthedocs documentation, as far as I can tell. If that is indeed required, please add them to the project's requirements.txt or pipfile so that Readthedocs can build correctly.