copyrights: update all files with license headers for 2021

- [x] add `concretize.lp`, `spack.yaml`, etc. to licensed files
- [x] update all licensed files to say 2013-2021 using
      `spack license update-copyright-year`
- [x] appease mypy with some additions to package.py that needed
      for oneapi.py

.github/actions/add-maintainers-as-reviewers/entrypoint.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 #
-# Copyright 2013-2020 Lawrence Livermore National Security, LLC and other
+# Copyright 2013-2021 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)