AST-1584 Use sphinx from Ubuntu instead of PyPi
The pages CI job at the SKAO mirror started failing recently due to issues with gcovr.
It appeared that the PyPi version of Sphinx installs a newer version of Jinja2, which is incompatible with gcovr from Ubuntu. Installing the Ubuntu python3-sphinx packages solves the issue.
https://gitlab.com/ska-telescope/external/sdp-dp3-visibility-preprocessing/-/pipelines/1400673038 shows the SKAO CI pipeline is green again with these changes.