Use Python 3 by default (#5)

Author: huan

.gitpod.yml

@@ -1,4 +1,5 @@
 tasks:
-  - init: make
+  - init: pyenv global $(pyenv versions | grep -Po '\d.\d.\d' | grep ^3)
+  - command: make
 image:
   file: .gitpod.Dockerfile