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`)

bin/spack

@@ -46,14 +46,6 @@ sys.path.insert(0, SPACK_LIB_PATH)
 SPACK_EXTERNAL_LIBS = os.path.join(SPACK_LIB_PATH, "external")
 sys.path.insert(0, SPACK_EXTERNAL_LIBS)
 
-import warnings
-# Avoid warnings when nose is installed with the python exe being used to run
-# spack. Note this must be done after Spack's external libs directory is added
-# to sys.path.
-with warnings.catch_warnings():
-    warnings.filterwarnings("ignore", ".*nose was already imported")
-    import nose
-
 # Quick and dirty check to clean orphaned .pyc files left over from
 # previous revisions.  These files were present in earlier versions of
 # Spack, were removed, but shadow system modules that Spack still