Include license in setup.py (#52)

Fix license tag and include python 3.9
Fixes #51

Author: sciencewhiz

setup.py

@@ -28,6 +28,7 @@
     long_description=long_description,
     long_description_content_type="text/markdown",
     url="https://github.com/wpilibsuite/sphinxext-opengraph",
+    license="LICENSE.md",
     install_requires=["sphinx>=2.0"],
     packages=["sphinxext/opengraph"],
     classifiers=[
@@ -36,12 +37,13 @@
         "Environment :: Web Environment",
         "Framework :: Sphinx :: Extension",
         "Intended Audience :: Developers",
-        "License :: OSI Approved :: MIT License",
+        "License :: OSI Approved :: BSD License",
         "Natural Language :: English",
         "Operating System :: OS Independent",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Programming Language :: Python",
         "Topic :: Documentation :: Sphinx",
         "Topic :: Documentation",