Add the current license header to files that do not have the header. Fix the license header in files if the text does not match.

Update the test_content_version submodule to pull the latest commit with the Apache 2.0 license header added to files.

Replace the custom Apache 2.0 license header to the standard Apache 2.0 license header in code files.

Add the hacking module to check code files for the standard Apache 2.0 license header. Update the flake8 config file and disable all other hacking checks.

Add copyright header above license header on all code files.

Add flake8-copyright to test requirements and configure the copyright check in the flake8 configuration file.

Pin version for flake8-copyright and hacking to avoid unexpected conflict due to version changes.