Include all license files in distributions

Without this patch, `THIRD_PARTY_LICENSES.md` was not being included in
the distribution files built by `setup.py`.

References:
* https://packaging.python.org/en/latest/guides/using-manifest-in/

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

Author: ajnelson-nist

setup.cfg

@@ -13,6 +13,9 @@ classifiers =
     License :: OSI Approved :: Apache Software License
     Operating System :: OS Independent
     Programming Language :: Python :: 3
+license_files =
+    LICENSE
+    THIRD_PARTY_LICENSES.md
 
 [options]
 include_package_data = true