platform.machine() gives the wrong answer if you're running 32-bit
Python on a 64-bit machine.

The reason we don't use platform.architecture() here is that it's not
reliable on macOS. See https://stackoverflow.com/a/1405971/823869.
Similarly, sys.maxsize is not reliable on Windows. See
https://stackoverflow.com/questions/1405913/how-do-i-determine-if-my-python-shell-is-executing-in-32bit-or-64bit-mode-on-os/1405971#comment6209952_1405971
and https://stackoverflow.com/a/3411134/823869.

Also use CARGO_CFG_TARGET_POINTER_WIDTH rather than inferring the Rust
target pointer width from CARGO_CFG_TARGET_ARCH.

… tests

Same reasoning as the previous commit.