Support Python 3.10

[kclowes]

What is wrong?

py-evm doesn't support Python 3.10. There was a change in the C API in Python 3.10 that we're running into. From the docs:

The PY_SSIZE_T_CLEAN macro must now be defined to use PyArg_ParseTuple() and Py_BuildValue() formats which use #: es#, et#, s#, u#, y#, z#, U# and Z#.]

Here is a link to the failed test run. The error message is: SystemError: PY_SSIZE_T_CLEAN macro must be defined for '#' formats.

How can it be fixed

I believe this change needs to happen in https://github.com/ethereum/ethash where the mkbytes_cache function is defined. Then we'll need a release and to pull the release in here.