py: memoryview: implement memoryview.cast if CPYTHON_COMPAT

[jepler]

This allows the following Python3-compatible code to work:

>>> b = bytearray([0] * 4)
>>> memoryview(b).cast('H')[0] = 0xffff
>>> b
bytearray(b'\xff\xff\x00\x00')

At one point I thought I might need this for my neopixel bit-twiddling, but I don't specifically need it right now. However, I wanted to post it up as a PR rather than just forgetting about it.

[dhalbert]

I did a brief test, and it did what I expected. I had no idea memoryview.cast() existed.