Fetch Permit2 allowance, expiration and nonce

[Elnaril]

Add a method RouterCodec.fetch_permit2_allowance() that would request the Permit2 contract and return a tuple (allowance, expiration, nonce) for a given (owner, token, spender).

Must be callable for a specific BlockIdentifier but call the latest block by default.

Constants must be defined in _constants.py.

As a parameter of this new method, the default Permit2 address would be 0x000000000022D473030F116dDEE9F6B43aC78BA3 as a ChecksumAddress, but users could call the method with a custom Permit2 address.

As part of the API exposed by this lib, this new method must have a docstring explaining its purpose, parameters and outputs.

Unit tests must be added to keep coverage at 100%

All integration tests where permit2 allowance is requested must be updated with this new method.

The doc (README.md) must be updated to explain the purpose of this new method and illustrate its usage and results.

[Elnaril]

Done with PR #62