Remove license classifier

Modern setuptools wants us to use the 'license'
argument now rather than a classifier.

Author: benmwebb

setup.py

@@ -44,9 +44,9 @@
       ext_modules=mod,
       packages=['ihm', 'ihm.util'],
       install_requires=['msgpack'],
+      license='MIT',
       classifiers=[
           "Programming Language :: Python :: 3",
-          "License :: OSI Approved :: MIT License",
           "Operating System :: OS Independent",
           "Intended Audience :: Science/Research",
           "Topic :: Scientific/Engineering",