add verifyingContract parameter to create permit2 signed message.

[freereaper]

dex on other chain use different permit2 address.

[NBFinanceTech]

Hello
line 78, the comma "，" is incorrect ,

[freereaper]

thanks, have fixed the typo.

[Elnaril]

Thank you for your PR!
Indeed, it make sense to have the verifyingContract configurable for UR/Permit2 forks.

Your changes look rather good to me, though a few refinements would be necessary to keep the lib consistent:

• as a parameter, verifyingContract should follow the PEP 8 naming convention and be verifying_contract. (of course the corresponding key in the structured_data dict remains verifyingContract! :) )
• verifying_contract should be a ChecksumAddress, not just a str
• the permit2 address should be defined with the other constants in _constants.py and imported into router_codec.py, something like:

# in _constants.py
from web3 import Web3

permit2_address = Web3.to_checksum_address("0x000000000022D473030F116dDEE9F6B43aC78BA3")

# and in `_structured_data_permit`
'verifyingContract': permit2_address,

• this new verifying_contract parameter should be documented in the method docstring. Basically, a short sentence to explain what it is and what is its default value.

Also, make sure the changes pass all the tests, including the linting.

Thanks again!

[freereaper]

Thanks for your time, I have submitted a new PR.

[Elnaril]

Closing because done by #52