Support for Ledger hardware wallets

### What was wrong?

Hardware wallets are generally considered the *gold* standard for private key security. Supporting them in web3.py would allow users to take advantage of that security.

### How can it be fixed?

Figure out how to do signing with a ledger hardware wallet.

https://github.com/LedgerHQ/blue-loader-python

This repository from the Ledger github is probably a good starting point.

### Definition of Done

- Something which conforms to the `LocalAccount` API used by `web3.eth.account`.
- An *easy* way to detect a connected to a ledger wallet.
- Should work for Nano-S
- Support for Blue not required but encouraged.
- Integration test suite which can be run with a connected ledger device.