def update_version(version_file, version_str):
    """update the version file."""
    prefixed_warning = """""\"!!!AUTOMATICALLY GENERATED!!! DO NOT EDIT.
see root thedropin/release.py.""\"
"""
    version_doc = f'{prefixed_warning}__version__ = "{version_str}"\n'
    with open(version_file, 'w') as f:
        f.write(version_doc)
