Python default version to python 3.8
Python default version to python 3.8

This commit updates the version to 3.8 so all committers use the
same python version.

thumbor/thumbor,kkopachev/thumbor,gi11es/thumbor,scorphus/thumbor,thumbor/thumbor,thumbor/thumbor,scorphus/thumbor,gi11es/thumbor,kkopachev/thumbor,scorphus/thumbor,kkopachev/thumbor,thumbor/thumbor,scorphus/thumbor,kkopachev/thumbor,gi11es/thumbor