docs: update "Deploying on Google App Engine"
docs: update "Deploying on Google App Engine"

Use the documentation for Python 3 instead of Python 2, which is deprecated.
fkazimierczak/flask,drewja/flask,fkazimierczak/flask,drewja/flask,mitsuhiko/flask,pallets/flask,mitsuhiko/flask,drewja/flask,pallets/flask,fkazimierczak/flask,pallets/flask