As the default Docker image support for Pegleg is using py36 as a base
image, this patch set simply removes the support for py35. This allows
us to use py36 features such as secrets and f-strings without needing to
cope with backward compatibility.
Change-Id: Ic156ca67d2a0fe6fdc74c2c9da253a1cb8a3f456
Signed-off-by: Tin Lam <tin@irrational.io>