Fix docs and docs-linkcheck tox targets:
It looks like recently sphinx began to check if footnotes are
referenced. If not, it issues a warning which is treated as error.
Fix Python 3.10 and 3.11:
It looks like opendev switched to Noble Ubuntu release.
Pin these versions to previous one.
Shift flake8 version.
New version ignores syntax whithin strings.
Fix formatting errors reported by flake8.
Change-Id: I9b23cccf0dc83fb56083b5fe8c9c026f5dff7e9a
643a58e255