compressor: use PyObject_TypeCheck instead of PyObject_IsInstance
compressor: use PyObject_TypeCheck instead of PyObject_IsInstance

This is how CPython does it most of the time.
terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard