force python 3.7.3 for travis ci

Author: Jerry Liang

.travis.yml

@@ -9,7 +9,7 @@ matrix:
       os: windows           # Windows 10.0.17134 N/A Build 17134
       language: shell       # 'language: python' is an error on Travis CI Windows
       before_install:
-        - choco install python3
+        - choco install python3 --version 3.7.3
         - pip install virtualenv
         - virtualenv $HOME/venv
         - source $HOME/venv/Scripts/activate