Fix F401 errors from flake8

`flake8` emits warnings for names that are imported but not used. In most cases, these either look like copy/pasted import blocks, or names left in from debugging or after refactoring. Common unused names are the `sys` or `time` modules, and base classes. Some modules were doing an import dance to get `simplejson` or standard `json`, but those modules no longer parse json directly, likely as the result of refactoring it to a lower level.

The `libcloud.utils.py3` module contains a lot of these unused imports, but that's intentional. Because of that, it includes a `flake8: noqa` comment near the top of the file to tell `flake8` to ignore it.