loader: pywatchman appears to already be py3 compatible

Our loader was doing some confusing things in pywatchman, but it looks
like we shouldn't be using it there anyway.

mercurial/__init__.py

@@ -34,6 +34,9 b' if sys.version_info[0] >= 3:'
             # zstd is already dual-version clean, don't try and mangle it
             if fullname.startswith('mercurial.zstd'):
                 return None
+            # pywatchman is already dual-version clean, don't try and mangle it
+            if fullname.startswith('hgext.fsmonitor.pywatchman'):
+                return None
 
             # Try to find the module using other registered finders.
             spec = None