Import submodules of importlib properly

[cjwatson]

As reported in https://bugs.debian.org/1118055, import importlib doesn't guarantee that e.g. importlib.machinery.SOURCE_SUFFIXES is visible. This is often masked by things like .pth files that automatically import bits of importlib, but in a sufficiently-minimal environment this may not be the case. Explicitly importing the necessary submodules is safer.

[lextm]

This repo is dead. You can read https://github.com/etingof/pysnmp/issues/429 and see what you might want to do.