py3: update output of check-py3-compat.py with python3
Recent work on mercurial/pure/mpatch.py made it import
mercurial.policy, and changed output of check-py3-compat.py with
python3 in test-check-py3-compat.t. But test-check-py3-compat.t isn't
yet updated.
mpatch: raise MemoryError instead of mpatchError if lalloc() failed
MemoryError is handled differently in dispatch._runcatch().
Since mpatch_errors[] isn't that useful now, I've changed it to a simple
switch statement.