Update externals to work with Python 3

- Update YAML version to support Python 3
- Python 3 support for ordereddict backport
- Exclude Python3 YAML from version tests.
- Vendor six into Spack.
- Make Python version-check tests work with Python 3
- Add ability to add version check exceptions with '# nopyqver' line
  comments.

lib/spack/docs/conf.py

@@ -51,6 +51,10 @@
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 sys.path.insert(0, os.path.abspath('exts'))
 sys.path.insert(0, os.path.abspath('../external'))
+if sys.version_info[0] < 3:
+    sys.path.insert(0, os.path.abspath('../external/yaml/lib'))
+else:
+    sys.path.insert(0, os.path.abspath('../external/yaml/lib3'))
 sys.path.append(os.path.abspath('..'))
 
 # Add the Spack bin directory to the path so that we can use its output in docs.
@@ -110,13 +114,13 @@
 for line in fileinput.input('spack.rst', inplace=1):
     if handling_spack:
         if not line.startswith('    :noindex:'):
-            print '    :noindex: %s' % ' '.join(spack.__all__)
+            print('    :noindex: %s' % ' '.join(spack.__all__))
         handling_spack = False
 
     if line.startswith('.. automodule::'):
         handling_spack = (line == '.. automodule:: spack\n')
 
-    print line,
+    sys.stdout.write(line)
 
 # Enable todo items
 todo_include_todos = True