Work around Tox hanging with --installpkg sdist due to orphaned build backend

[vytas7]

This is an incomplete workaround for #3512 (which "works for me" though for all practical purposes) just to get the ball rolling.

See the discussion on the issue (#3512) for the detailed explanation what this workaround does.

• [x] ran the linter to address style issues (tox -e fix)
• [x] wrote descriptive pull request text
• [ ] ensured there are test(s) validating the fix
• [x] added news fragment in docs/changelog folder
• [ ] updated/extended the documentation