Add async eth.modify_transaction

[kclowes]

What was wrong?

Some methods are still unavailable on the AsyncEth class.

How can it be fixed?

Following the pattern already established, add modify_transaction to the AsyncEth class, and add corresponding tests. The AsyncHTTPProvider docs in docs/provider.rst will also need to be updated.

[ketan3108]

Hey @kclowes ,I am interested in contributing. Can you guide me by providing some references ?

[esing1201]

Hi @kclowes I am happy to claim this issue!

[kclowes]

Thanks @esing1201! Here is the most recent example of a new async function that's been merged: #2848. The replace_transaction PR might also be helpful: #2847.