Make setuptools license field SPDX-compliant

https://spdx.org/licenses/
https://spdx.github.io/spdx-spec/SPDX-license-expressions/#d42-disjunctive-or-operator

Author: pquentin

setup.py

@@ -22,7 +22,7 @@
     long_description_content_type='text/x-rst',
     author='Frazer McLean',
     author_email='[email protected]',
-    license='MIT -or- Apache License 2.0',
+    license='MIT OR Apache-2.0',
     packages=find_packages('src'),
     package_dir={'': 'src'},
     install_requires=['attrs>=19.2.0'],