BUG: The call to Pdb.set_trace occured in a conteext manager making rdb stop in
contextlib, not in the frame given.

The ignore_errno context manager there didn't suppress anything as
Pdb.set_trace does not raise an ECONNRESET in any case. After the
removal, the method was the same as Pdb.set_trace, so this just
removes Rdb's override.

The test was updated to reflect the fact that Pdb.set_trace will not
raise a ECONNRESET.
Merge pull request #2331 from llllllllll/rdb-stops-in-contextlib

rdb.set_trace stops in contextlib