Add source highlight package (#15473)

* add GNU source-highlight package

* fix flake8

* Allows the package to download from GNU mirrors

var/spack/repos/builtin/packages/source-highlight/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 SourceHighlight(AutotoolsPackage, GNUMirrorPackage):
+    """This program, given a source file, produces a document with syntax
+    highlighting. It also provides a C++ highlight library
+    (since version 3.0). """
+
+    homepage = "https://www.gnu.org/software/src-highlite/"
+    gnu_mirror_path = "src-highlite/source-highlight-3.1.8.tar.gz"
+
+    version('3.1.9', sha256='3a7fd28378cb5416f8de2c9e77196ec915145d44e30ff4e0ee8beb3fe6211c91')
+    version('3.1.8', sha256='01336a7ea1d1ccc374201f7b81ffa94d0aecb33afc7d6903ebf9fbf33a55ada3')
+
+    depends_on('boost')