convert: get test-convert-darcs.t passing under py3
With these changes it now passes locally for me.
The globals check and associated check in the test could probably go too (unless a distro is likely to rip out the ElementTree module?), but I figured that could be done later if need be.
This won't make a difference in CI though, since darcs is currently missing from the images.
Rebased-to-stable version of !169 (closed)