Fix AsyncEth module typing

[kclowes]

What was wrong?

Because the eth module is called eth regardless of whether or not it refers to the Eth module or the AsyncEth module, the typing is a little bit funky. Once AsyncEth is completely switched over to mirror the functionality of the Eth module, we can make the eth attr here be Union[AsyncEth, Eth]. Then we can go through and remove type: ignores throughout codebase on all await w3.eth... lines.

[fselmo]

closed by #2819