Use new license declaration format (#108)

[Contributed by @bkmgit, reviewed by Lydia]

Based on guidance about using a new format to specify the license information, https://packaging.python.org/en/latest/guides/licensing-examples-and-user-scenarios/.

Passed CI testing (but I wouldn't expect that to be where a problem would be encountered).

Author: lydia-duncan

setup.py

@@ -15,7 +15,8 @@
     version=version,
     url='https://github.com/chapel-lang/sphinxcontrib-chapeldomain',
     download_url='https://pypi.python.org/pypi/sphinxcontrib-chapeldomain',
-    license='Apache License v2.0',
+    license='Apache-2.0',
+    license_files=['LICENSE'],
     author='Chapel Team',
     author_email='[email protected]',
     description='Chapel domain for Sphinx',
@@ -27,7 +28,6 @@
         'Environment :: Web Environment',
         'Intended Audience :: Developers',
         'Intended Audience :: Education',
-        'License :: OSI Approved :: Apache Software License',
         'Operating System :: OS Independent',
         'Programming Language :: Python',
         'Programming Language :: Python :: 3',