Return |True| if *prs_part* is a valid main document part, |False|
    otherwise.

def _is_pptx_package(prs_part):
    """
    Return |True| if *prs_part* is a valid main document part, |False|
    otherwise.
    """
    valid_content_types = (
        CT.PML_PRESENTATION_MAIN,
        CT.PML_PRES_MACRO_MAIN,
    )
    return prs_part.content_type in valid_content_types