Compatibility with pcodec 0.3

[dstansby]

pcodec 0.3 was released recently, and it has breaking changes. For now I've pinned to pcodec<0.3 in #622, but we should probably update that to pcodec>=0.3 and adapt numcodecs to work with the newer version.

[jakirkham]

cc @rabernat (as I believe you added this codec)