tskit package (#19725)

* tskit package

* Update var/spack/repos/builtin/packages/tskit/package.py

I can't see any hard requirement for 3.6:

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

* fixes following PR review

* Update var/spack/repos/builtin/packages/tskit/package.py

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

Co-authored-by: Adam J. Stewart <ajstewart426@gmail.com>

var/spack/repos/builtin/packages/tskit/package.py

@@ -0,0 +1,24 @@
+# Copyright 2013-2020 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 Tskit(PythonPackage):
+    """The tskit library provides the underlying functionality used to load,
+    examine, and manipulate tree sequences"""
+
+    homepage = "https://tskit.readthedocs.io/en/latest/"
+    url      = "https://pypi.io/packages/source/t/tskit/tskit-0.3.1.tar.gz"
+
+    version('0.3.1', sha256='b9c5a9b2fb62a615e389036946345ef8a35b09f1ffee541995b16f97fedb3d36')
+
+    depends_on('python@3.6:',      type=('build', 'run'))
+    depends_on('py-setuptools',    type=('build', 'run'))
+    depends_on('py-svgwrite',      type=('build', 'run'))
+    depends_on('py-jsonschema',    type=('build', 'run'))
+    depends_on('py-h5py',          type=('build', 'run'))
+    depends_on('py-numpy@1.7:',    type=('build', 'run'))
+    depends_on('py-attrs@19.1.0:', type=('build', 'run'))