test against python3.9

https://github.com/fphammerle/ical2vdir/commit/d679a828a6de976c477ddfa7f3ced0b1be57fc3c

.github/workflows/python.yml

@@ -37,6 +37,7 @@ jobs:
         - 3.6
         - 3.7
         - 3.8
+        - 3.9
       fail-fast: false
     steps:
     - uses: actions/checkout@v1

setup.py

@@ -24,6 +24,7 @@ setuptools.setup(
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Topic :: Utilities",
     ],
 )