Choose a default backend in `next` branch

[fselmo]

We talked about defaulting to EELSBackend, which would keep all the existing conditionals for checking if py-evm is installed as an install extra. If this is desired, we should remove all the checks for eels_is_available(), etc, and require it as part of the install, leaving py-evm as an install extra.

If we want py-evm to be the default (I don't lean this way), then we should do the opposite and require py-evm as a dependency and leave in the eels_is_available() conditional checks.

We should also remove the is_supported_pyevm_version_available() check and just control this in the dependency pins. This seems like a stale check that is not doing anything useful.

[fselmo]

I vote to default to eels, py-evm optional 👀