Skip pyethash dependency in Windows

[carver]

• py-evm Version: 0.2.0a11
• OS: win

What is wrong?

pyethash won't build on Windows, and probably won't anytime soon. See Consensys/ethjsonrpc#5

The ethash code is C99, which is not well-supported under Windows.

This makes it impossible to install py-evm on Windows, as well as any package that depends on py-evm like the test suite in web3.py. (See ethereum/web3.py#166)

How can it be fixed

A couple options come to mind:

1. Skip the pyethash install on Windows, so that Windows users can install the EVM without mining capability
2. Move the pyethash dependency to an "extra" called something like mining.

Number 1 seems better, because it supports mining with a default installation, in non-Windows environments. Number 2 requires an install like pip install py-evm[mining] to support mining.

Does anyone see problems with 1 or 2, or a better solution?

[carver]

Does anyone see problems with 1 or 2

So, yeah, it's used for pow verification also. So... this is a no-go until pyethash (or some alternative) is made to work on Windows.