Do not namecheck modules that populate at runtime.

### Motivation
Some python modules have dynamic namespaces that are populated the moment they are imported. Pylint cannot statically-process these modules, so throws off false warnings. Right now, this affects `distutils` and `behave`, but probably other modules will also have the same problem in the future.

### Changelog
* Instruct `pylint` to not namecheck inside modules that populate namespaces at runtime.
* Drop some suppressed warnings; they aren't omitted any more.

### Notes
* This PR doesn't fix the import errors (F401), which should still be suppressed.
* Relevant discussion on the [pylint](https://bitbucket.org/logilab/pylint/issue/73/pylint-is-unable-to-import) bug tracker.

This PR is a prerequisite for successful integration of #897.