Add a base GitHub Action definition for continuous integration
for a Python package, inspired from
https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-python

Update the Python package CI with preliminary commands
for linting with pylint and reporting test coverage with pytest-cov.
Also define the job matrix to test on both macos and ubuntu.

Fix the Python package CI to trigger on the default branch and
to not fail a run after reporting lint errors.
Also remove testing for the macos environment for now.

Add Brewfile for PyICU dependency resolution for macos
and the macos environment back as a CI environment.

Use GitHub action to setup Homebrew for ubuntu runner image,
as described in actions/runner-images#6283