CI: test on pypy-3.8 also

[ThomasWaldmann]

No description provided.

[ThomasWaldmann]

Doesn't work with pypy:

ImportError while importing test module '/home/runner/work/deflate/deflate/tests/test_compress.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/opt/hostedtoolcache/PyPy/3.8.12/x64/lib/pypy3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/test_compress.py:6: in <module>
    import deflate
E   ImportError: /home/runner/work/deflate/deflate/deflate.pypy38-pp73-x86_64-linux-gnu.so: undefined symbol: Py_Initialize

[ThomasWaldmann]

can be tried again later, after fixing it for pypy.