Fix Windows path issue when importing interfaces

[iamdefinitelyahuman]

The Issue

To prevent imports outside of the the root path, vyper-json prepends import paths with /__vyper and then ensures the final resolved path is still a subfolder of that path. On Windows this check is failing with ValueError: 'C:\\__vyper\\path.vy' does not start with '\\__vyper'. Oopsies.

You can see a detailed traceback for the issue here.

How I fixed it

Perform the relative_to check against a Path object, so that it works on any filesystem.

How to verify it

I confirmed that it works in a Windows 10 VM. Not sure if anyone wants to add Windows to the CI...

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[iamdefinitelyahuman]

Ignore my force pushes, was trying to get Windows CI to work but there's an upstream issue installing pyethash (Consensys/ethjsonrpc#5) which is required by py-evm (ethereum/py-evm#395). Unless we remove py-evm as a testing dependency, running tests on Windows is a no-go.

[fubuloubu]

What if we don't run the full test-suite with Windows, but instead run just the CLI stuff?