Add slither support to brownie mix

[saltyfacu]

See https://github.com/crytic/slither

More info:

• Add Support for Brownie crytic/slither#638
• Brownie does not generate compilation artifacts for dependencies eth-brownie/brownie#772

Copied from yearn/yearn-protocol#28 by milkyklim and comment by Macarse

[PatrickAlphaC]

Are we just adding it to brownie-mixes? It seems it would make sense to add to all of brownie no?

[fubuloubu]

Are we just adding it to brownie-mixes? It seems it would make sense to add to all of brownie no?

General brownie support precedes support for these repos

[numan]

Attempted to make some progress on this by addressing one of the big blockers:

eth-brownie/brownie#1321

Once it is merged, I should be able to create a PR for this issue.