Pylint Workflow Update

[hoangvu5]

In .github/workflows/pylint.yml:

1. Update actions/checkout@v2 and actions/setup-python@v2 to the latest version
2. Add pip dependency caching with cache: 'pip'
3. Add branch and path filters to only run on Python file changes
4. Rename job from build to lint (more descriptive)
5. Add support for pull requests

For testing:

1. Make a minor Python code change and push to a branch, verify that Pylint runs.
2. Make a minor non-Python change and push to a branch, verify that Pylint doesn't run.
3. Check that caching works (second run should be faster)