Seer - cow-py buy/sell token integration

[gabrielfior]

We will use Cowswap to swap outcomeTokens <> collateral.

• Add cowswap integration cow-py (https://github.com/cowdao-grants/cow-py)
• Allow buy/sell tokens using cow-py
• Write integration test for selling an outcome token of an open Seer market (make sure order is posted, use unrealistic price to avoid it being executed and we at some point running out of funds)

[gabrielfior]

Cowswap (see slack messages) has some constraints in-place with respect to which tokens can be traded. tl;dr not all outcome tokens (from the Swapr v3 pools) can be traded there.
Hence it looks like the easiest path forward is to trade directly on Swapr v3.