`from_bytes` does not properly release memory buffers under mmap

[poke1024]

OS: macOS-12.1-arm64-arm-64bit Python: 3.9.7

Running the demo.py from the following minimal example

pycapnp-mmap-issue.zip

will reliably produce the following exception 100% of the time:

Traceback (most recent call last):
  File "/Users/ch/Desktop/pycapnp-mmap-issue/demo.py", line 18, in <module>
    mv.release()
BufferError: memoryview has 1 exported buffer

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/ch/Desktop/pycapnp-mmap-issue/demo.py", line 18, in <module>
    mv.release()
BufferError: cannot close exported pointers exist

I strongly suspect this will happen on other platforms as well. Note that the same issue appears when not creating an explicit memoryview (see read2() in attached code).