dispatch: don't attempt to import debugger as bytestring

The __import__ thingie needs a string, not a bytestring. Guess I'm the
only one who uses this once in a while and noticed it was broken.

mercurial/dispatch.py

@@ -413,7 +413,7 b' def _runcatch(req):'
                     # debugging has been requested
                     with demandimport.deactivated():
                         try:
-                            debugmod = __import__(debugger)
+                            debugmod = __import__(pycompat.sysstr(debugger))
                         except ImportError:
                             pass  # Leave debugmod = pdb