Actually address the PY_SSIZE_T_CLEAN issue

[qistoph]

Current versions of python report warnings or even errors like this:

DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats

This PR users the required Py_ssize_t and sets the appropriate PY_SSIZE_T_CLEAN. For details see the first not at https://docs.python.org/3/c-api/arg.html#arg-parsing.

Fixes #5
Improvement on #7, which only sets PY_SSIZE_T_CLEAN, but doesn't actually address the issue.

Also updates the version to 0.1.2.

[gtrafimenkov]

Hello @qistoph

Thank you for the PR. Unfortunately I don't have capacity for maintaining this repository. Feel free to fork if you want.