Favor nonce from Safe Multisig api

[fedgiac]

If there is a pending transaction in the Safe, the self-sell script overwrites it when creating the settle transaction.
With these changes, the created transaction will use the nonce that is recommended by the Safe API. This is the same nonce used by the interface to determine the nonce for the next transaction.

I also sneaked in a change in the parameters, since the script is not working unless --to-token is explicit.

Test Plan

This is difficult to test as the script only works on mainnet (see #74).
I suggest to see that the api call returns the expected nonce.
For example, 0xA03be496e67Ec29bC62F01a428683D7F9c204930 is a Safe that right now has a pending transaction, and you can see that the API output is the expected one:

curl -X 'GET' \
  'https://safe-client.safe.global/v1/chains/11155111/safes/0xA03be496e67Ec29bC62F01a428683D7F9c204930/nonces' \
  -H 'accept: application/json'
{"currentNonce":10,"recommendedNonce":11}

[coveralls]

Pull Request Test Coverage Report for Build 8095554159

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.398%

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Totals
Change from base Build 8048270935:	0.0%
Covered Lines:	364
Relevant Lines:	366

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💛 - Coveralls

[fleupold]