WIP: Enable PyPy

[jakirkham]

Add PyPy 3.9 to CI.

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[jakirkham]

There are a couple test failures, but surprisingly not that many

=================================== FAILURES ===================================
______________________________ test_encode_decode ______________________________

    def test_encode_decode():
        for arr, codec in itertools.product(arrays, codecs):
>           check_encode_decode(arr, codec)

numcodecs/tests/test_lzma.py:53: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
numcodecs/tests/common.py:59: in check_encode_decode
    compare_arrays(arr, dec, precision=precision)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

arr = array([  0,   1,   2,   3,   4,   5,   6,   7,   8,   9,  10,  11,  12,
        13,  14,  15,  16,  17,  18,  19,  20,...1, 982, 983, 984, 985, 986, 987,
       988, 989, 990, 991, 992, 993, 994, 995, 996, 997, 998, 999],
      dtype=int32)
res = array([  0,   1,   2,   3,   4,   5,   6,   7,   8,   9,  10,  11,  12,
        13,  14,  15,  16,  17,  18,  19,  20,..., 232, 233,
       234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246,
       247, 248, 249], dtype=int32)
precision = None

    def compare_arrays(arr, res, precision=None):
    
        # ensure numpy array with matching dtype
        res = ensure_ndarray(res).view(arr.dtype)
    
        # convert to correct shape
        if arr.flags.f_contiguous:
            order = 'F'
        else:
            order = 'C'
>       res = res.reshape(arr.shape, order=order)
E       ValueError: cannot reshape array of size 250 into shape (1000,)

numcodecs/tests/common.py:34: ValueError
_______________________________ test_encode_none _______________________________

    def test_encode_none():
        a = np.array([b'foo', None, b'bar'], dtype=object)
        codec = VLenBytes()
>       enc = codec.encode(a)

numcodecs/tests/test_vlen_bytes.py:85: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   store_le32(<uint8_t*>data, l)
E   TypeError: expected bytes, NoneType found

numcodecs/vlen.pyx:250: TypeError

---------- coverage: platform linux, python 3.9.18-final-0 -----------
Coverage XML written to file coverage.xml

=========================== short test summary info ============================
FAILED numcodecs/tests/test_lzma.py::test_encode_decode - ValueError: cannot reshape array of size 250 into shape (1000,)
FAILED numcodecs/tests/test_vlen_bytes.py::test_encode_none - TypeError: expected bytes, NoneType found
======= 2 failed, 582 passed, 35 skipped, 36 xfailed in 75.66s (0:01:15) =======