Remove Python 3.9 classifier from setup.cfg (#639)

Author: adamchainz

setup.cfg

@@ -31,7 +31,6 @@ classifiers =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
-    Programming Language :: Python :: 3.9
     Topic :: Database
 license_file = LICENSE