Match python/convertrules.py versions to that of stable branch.

Match python/convertrules.py versions to that of stable branch.

This became necessary due to cherry-picking the following into stable:

convert-ly rule for new make-relative semantics accepting music arguments

This only works when the "reference pitch" is the first or last of
the list.

No attempt is made to rewind the version numbers of already converted
files from 2.19.0 to 2.17.97.