setup.py: add MIT license to classifiers

The project contains an MIT license in the LICENSE file, but this is not
reflected in the package classifiers.

setup.py

@@ -18,6 +18,7 @@ setup(
     classifiers=[
         "Topic :: Communications :: Email",
         "Operating System :: OS Independent",
+        "License :: OSI Approved :: MIT License",
 
         "Programming Language :: Python :: 3",
         "Programming Language :: Python :: 3.6",