ci: remove python 3.5 from build matrix

Python 3.5 support was dropped for ptb 13.0: https://github.com/python-telegram-bot/python-telegram-bot/wiki/Transition-guide-to-Version-13.0#python-35

Also python 3.9 was added to the build matrix: python-telegram-bot/python-telegram-bot#2132

Author: devcarron

.github/workflows/python-lint-test.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.5', '3.6', '3.7', '3.8']
+        python-version: [ '3.6', '3.7', '3.8', '3.9' ]
 
     steps:
     - uses: actions/checkout@v2