Fix issues related to the switchover to pytest. (#2685)

- Remove stale reference to `import nose` from `bin/spack` script.
- Add `py` to externals (missing dependency for `pytest`)

lib/spack/external/__init__.py

@@ -35,12 +35,15 @@
 
     jsonschema:  An implementation of JSON Schema for Python.
 
-    pytest:      Testing framework used by Spack.
-
     ordereddict: We include our own version to be Python 2.6 compatible.
 
+    py:          Needed by pytest.  Library with cross-python path,
+                 ini-parsing, io, code, and log facilities.
+
     pyqver2:     External script to query required python version of
                  python source code. Used for ensuring 2.6 compatibility.
 
+    pytest:      Testing framework used by Spack.
+
     yaml:        Used for config files.
 """