Provide methods to save and restore state of the global ETSConfig

[mdickinson]

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.)

[mdickinson]

We could even provide a context manager that saves and restores state, similar in spirit to something like warnings.catch_warnings.