Improve the `pyconvert` rule dispatch system

**Is your feature request related to a problem? Please describe.**
I add a rule with a bug in the function, try a conversion, see the bug, and define a new rule with a different (hopefully less buggy) function. However, when I try the conversion again, I get the old function :cry:.

**Describe the solution you'd like**
When tied for priority in all other respects, I'd like to get the most recently added conversion, not the least recently added conversion.

**Describe alternatives you've considered**
Removing rules is a more robust approach, and works even if I added a new rule with an erroneously high priority, but that can be orthogonal to this feature request.

**Additional context**
I'm happy to make a PR if this is a change we want to make. It's technically breaking, but only barely, and IIUC we are okay with more breaking releases at this time.