ci: add python 3.9

Author: chenrui333

.travis.yml

@@ -5,8 +5,10 @@ addons:
     packages:
     - snapcraft
 python:
-- '3.7'
-- '3.8'
+  - '3.7'
+  - '3.8'
+  - '3.9'
+
 env: LIBGIT2=~/libgit2/_install/ LD_LIBRARY_PATH=~/libgit2/_install/lib
 before_install: ./.travis.sh
 install: pip install -r requirements.txt .