Add wZEC pool; brownie-token-tester integration

[samwerner]

What I did

This PR consists of the following changes:

• Add a StableSwap base pool for wZEC/renZEC
• Integrate the brownie-token-tester into the test suite to allow for easy minting of tokens for testing
• Bump isort (removes unknown_third_party issues) and flake8 versions

Note

• Contracts are not deployed yet

How to verify it

Integation of brownie-token-tester

Run full test suite: brownie test

Test wZEC Pool

Run test suite: brownie test --pool wzec

Run test suite against forked mainnet: brownie test --network mainnet-fork --pool wzec