Implement client for Southwest Power Pool

[r24mille]

The SPPClient is listed as a balancing authority in the client_factory but the class does not implement any of the abstract methods required by BaseClient. Similarly, the test_spp.py class is empty. This dead code is technical debt unless we actually implement a client for Southwest Power Pool.

Their portal at https://marketplace.spp.org/ seems like they should have everything we need. We just need to parse it.

[r24mille]

Upon looking into it, most of the SPP client was lost with rather heavy-handed commit 7ba5aa1 to remove a selenium dependency. Attempt to restore the client without the dependency on selenium. There has to be a lighter-weight way to make the reqests, but all the response parsing functionality should still be valid.

[ajdonnison]

It looks like it was introduced here: fa15a01a with a comment suggesting it probably never worked. I guess we need to do some work on that.