When trying to use the hg-git extension to mercurial on OSX Catalina I got:
*** failed to import extension hggit from /foo/bar/dev/hg-git/hggit: No module named dulwich.errors
To fix, I installed the python 2 dulwich via pip. I then copied the python 2.7 dulwich package from my main site-packages to the homebrew hg site-packages:
Then everything worked fine in the convert. All the other instructions online were too complicated.