[Backport 2.28] Re-add python install of requirements to Travis CI. #8254
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Description
Re-add the python dependancies installation to Travis CI, also correct 'distro to 'dist' and update ubuntu to focal in line with previous Travis settings. Too much got removed when disabling travis and the Coverity scan build was failing on 'make generated-files', due to lack of installed python dependencies.
Trivial backport of #8253
PR checklist
Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")
provided, ornot required (no user facing change)provided, ornot required