fix deprecated parameter license_file

setup.cfg

@@ -3,7 +3,7 @@ name = solarmach
 author = Jan Gieseler
 author_email = jan.gieseler@utu.fi
 license = BSD 3-clause
-license_file = licenses/LICENSE.rst
+license_files = licenses/LICENSE.rst
 url = https://github.com/jgieseler/solarmach
 description = Multi-spacecraft longitudinal configuration plotter
 long_description = file: README.rst