Updating the Read-The-Docs configuration.
Updating the Read-The-Docs configuration.

RTD recommends pinning the sphinx version via the requirements.txt
file
(https://docs.readthedocs.io/en/stable/builds.html#understanding-what-s-going-on).
We weren't doing this before.

richardbeare/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,SimpleITK/SimpleITK,InsightSoftwareConsortium/SimpleITK,richardbeare/SimpleITK,blowekamp/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,richardbeare/SimpleITK,SimpleITK/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,SimpleITK/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK