Vyper support is limited

[Gustavo Grieco]

Echidna works on vyper contracts running the .vy files directly, however:

• properties should be hardcoded in the contract
• any use of framework will fail to work (e.g. brownie).
• slither won't run on it, so there is no constant extraction or assertion detection.
• we still get the constants if they are returned by a function inside the code
• coverage feedback and source code integration works

It is expected to have a limited effectiveness since we cannot get useful constants from the source code.

[Gustavo Grieco]

This is something we won't fix unless Vyper has more traction.