Use classifiers to specify the license. (#185)

Classifiers are a standard way of specifying a license, and make it easy
for automated tools to properly detect the license of the package.

The "license" field should only be used if the license has no
corresponding Trove classifier.

setup.py

@@ -45,11 +45,11 @@
     long_description_content_type='text/markdown',
     tests_require=['flake8', 'pytest>=4.1'],
     keywords='probabilistic machine learning bayesian statistics',
-    license='MIT License',
     classifiers=[
         'Intended Audience :: Developers',
         'Intended Audience :: Education',
         'Intended Audience :: Science/Research',
+        'License :: OSI Approved :: MIT License',
         'Operating System :: POSIX :: Linux',
         'Operating System :: MacOS :: MacOS X',
         'Programming Language :: Python :: 3.6',