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

@@ -12,7 +12,7 @@
     long_description=open("README.rst").read(),
     author="Emmanuel Leblond",
     author_email="[email protected]",
-    license="MIT -or- Apache License 2.0",
+    license="MIT OR Apache-2.0",
     packages=find_packages(),
     entry_points={'pytest11': ['trio = pytest_trio.plugin']},
     install_requires=[