I would suggest the following patch, can someone with a macos check if
the problem is still present and if yes, test the following patch?

--- a/roundup/backends/__init__.py
+++ b/roundup/backends/__init__.py
@@ -67,6 +67,9 @@ def have_backend(name):
         for name in _modules.get(name, (name,)):
             if str(e).startswith('No module named %s'%name):
                 return 0
+            myslerr = 'Cannot load MySQL client libraries'
+            if name == 'mysql' and str(e).startswith(myslerr):
+                return 0
         raise
     return 0