imports: fixed usage of exceptions in import statements as it's reserved in python3

vcsserver/hgpatches.py

@@ -63,7 +63,7 b' def _dynamic_capabilities_wrapper(lfprot'
 def patch_subrepo_type_mapping():
     from collections import defaultdict
     from hgcompat import subrepo
-    from exceptions import SubrepoMergeException
+    from vcsserver.exceptions import SubrepoMergeException
 
     class NoOpSubrepo(subrepo.abstractsubrepo):