setup.py: Add trove classifier for Python 3.9

Add support for Python 3.9 to https://pypi.org/project/prompt-toolkit/

Tests have been passing on Python 3.9
https://github.com/prompt-toolkit/python-prompt-toolkit/blob/master/.github/workflows/test.yaml#L13

Author: cclauss

setup.py

@@ -50,6 +50,7 @@ def get_version(package):
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: 3 :: Only",
         "Programming Language :: Python",
         "Topic :: Software Development",