fix: don't clobber a Python exception on COPY FROM with QueryCanceled

fix: don't clobber a Python exception on COPY FROM with QueryCanceled

We trigger the server to raise the QueryCanceled; however, the original
exception has more information (the traceback). We can consider the
server exception just a notification that cancellation worked as
expected.

This is a mild change in behaviour, as the fixed tests state. However,
raising QueryCanceled is not explicitly documented and not part of a
strict interface, so we can probably change the exception raised without
needing to wait for psycopg 4.

Close #593