Release the GIL during decompression

[brianhelba]

This requires wrapping code in Py_BEGIN_ALLOW_THREADS: https://docs.python.org/3/c-api/init.html#releasing-the-gil-from-extension-code

As part of this, memory allocation (including the use of Bytes objects) will need to be switched to use exclusively PyMem_RawMalloc, as PyMem_Malloc and PyBytes_* operations (which internally use PyObject_Malloc) are not safe to be used without the GIL: https://docs.python.org/3/c-api/memory.html