Proposal how to proceed with multi-Python support and embedded Python (fabio, bumps)

The multipython merge was too big, insufficiently reviewed, and not production ready. We need to go back to status quo ante, and proceed in smaller steps.

• Reset main to commit 30ced44c of 9feb22.
• Delete issues #215 (check) #211 #209 #201 #199 #198 #196 #189.
• Reopen issues #213 (closed) #212 (closed) #207 (closed) #200 (closed) #194 (closed) #193 (closed)
• Cherry-pick everything from current main that is not related to multipython.
• Extend CI to multiple Py versions, using !691 (closed) and parts of current main.
• Cherry-pick code needed for using fabio or bumps (low priority: after next release).