Fix deprecate warning

[OldPanda]

It complains with the following warning when running under Python 3.8+

Python 3.8.6 (v3.8.6:db455296be, Sep 23 2020, 13:31:39)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import mmh3
>>> mmh3.hash("hello")
<stdin>:1: DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats
613153351

which was added in Python 3.8 source code.