Provide methods to save and restore state of the global ETSConfig

For testing purposes, it's often convenient to make temporary changes to the state of the global `ETSConfig` object and then undo those changes in the test cleanup. This generally involves digging into the `ETSConfig` internals.

This could be made easier and less fragile with the addition of `getstate` and `setstate` methods on `ETSConfig`. (Or possibly a `state` property that's readable and writeable.)