Drop 3.3 support.

3.3 importlib sucks.
Update namespaces of dg modules instead of extending `builtins`.

Cons:
  * harder (impossible?) to retroactively replace these built-ins;
  * somewhat pollutes namespaces (though `import` does the same anyway).

Pros:
  * no more on-shutdown exceptions due to `builtins` being reset to its original state;
  * more explicit;
  * does not affect Python code.