old_contents: 
  from req_install import InstallRequirement
  from req_set import RequirementSet, Requirements
  from req_file import parse_requirements
  
  __all__ = [RequirementSet, Requirements, InstallRequirement,
             parse_requirements]
  
new_contents: 
  from .req_install import InstallRequirement
  from .req_set import RequirementSet, Requirements
  from .req_file import parse_requirements
  
  __all__ = [RequirementSet, Requirements, InstallRequirement,
             parse_requirements]
  
subject: use explicit relative imports for Python 3