ipdb can now be run without pip installing. (#4257)

docs/contributing.rst

@@ -69,10 +69,8 @@ either in the same directory as ``foo.py`` or in the ``tests`` subdirectory
 (if there isn't, make one). While you are working on your code and tests, run
 ``python foo_test.py`` to run the relevant tests.
 
-For debugging, we recommend running ``pip install ipdb`` and putting
-``import ipdb; ipdb.set_trace()`` statements inside the source
-code. Alternatively, you can use Python's standard library `pdb`,
-but you won't get TAB completion.
+For debugging, we recommend putting
+``import ipdb; ipdb.set_trace()`` statements inside the source code.
 
 Once you are done with your code changes, and the tests in ``foo_test.py`` pass,
 run all of the unittests for Certbot with ``tox -e py27`` (this uses Python