restore sdist/install step during sphinx test

Author: jesterhazy

tox.ini

@@ -92,8 +92,6 @@ commands =
 
 [testenv:sphinx]
 basepython = python3
-skipdist = true
-skip_install = true
 changedir = doc
 # Based on: https://github.com/rtfd/readthedocs.org/blob/8f0c78dde5edcc85acf90462a8518735a25482d3/readthedocs/doc_builder/python_environments.py#L263
 install_command = python -m pip install --upgrade -I {packages}