new package: py-backports-weakref (#12262)

* new package: py-backports-weakref

* use the version at pypi.org

var/spack/repos/builtin/packages/py-backports-weakref/package.py

+# Copyright 2013-2019 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class PyBackportsWeakref(PythonPackage):
+    """Backports of new features in Python's weakref module"""
+
+    homepage = "https://github.com/PiDelport/backports.weakref"
+    url      = "https://pypi.org/packages/source/b/backports.weakref/backports.weakref-1.0.post1.tar.gz"
+
+    version('1.0.post1', sha256='bc4170a29915f8b22c9e7c4939701859650f2eb84184aee80da329ac0b9825c2')
+
+    depends_on('py-setuptools', type='build')