Subvertpy is getting increasingly more difficult to install and use, as

it's no longer even compiling with a default compiler of clang and

modern Subversion. There hasn't been a release since January 2021. And

on top of that, our https://pysvn.reviewboard.org installer works pretty

well.
This drops the development dependency and removes its recommendation

from the manual.
Links have also been updated in the Manual to point to modern locations

for Subversion and PySVN.

In a new environment, unit tests pass (with subvertpy tests skipping).
Built the manual and saw the new text and URLs.