Add mechanism and scripts for building and uploading the Python distribution
package `tflite_micro` to PyPI. These scripts are intended mainly for use by CI
when generating packages for distribution via for PyPI, and won't be used by
most developers. Building a package for local use is still done via a normal
Bazel build.
Heavily comment the scripts with rationale and technical details of the
implementation.
Make significant updates to python/tflite_micro/README.md which explain
building, installing, and uploading the package to PyPI. Leave some cleanup of
the existing text for later.
Add a build setting `--//python/tflite_micro:compatibility_tag` for setting
:whl's platform compatibility tag. Unfortunately, it cannot derived
automatically from the execution environment by the current implementation of
@rules_python.
BUG=part of #1484
