Link Python 3 doc instead of Python 2 (#1153)

Author: pushfoo

source/conf.py

@@ -416,7 +416,6 @@
     'pypa': ('https://www.pypa.io/en/latest/', None),
     'python': ('https://docs.python.org/3', None),
     'python-guide': ('https://docs.python-guide.org', None),
-    'python2': ('https://docs.python.org/2', None),
     'setuptools': ('https://setuptools.readthedocs.io/en/latest/', None),
     'spack': ('https://spack.readthedocs.io/en/latest/', None),
     'sphinx': ('https://www.sphinx-doc.org/en/master', None),