input: Write a python function to compress files using gzip and shutil libraries.
  assert compress_files('/files', '/files/compressed', 3) == '/files/compressed.gz'
  assert compress_files('/my_data', '/my_data/compressed', 5) == '/my_data/compressed.gz'
  assert compress_files('/documents', '/documents/compressed', 1) == '/documents/compressed.gz'
output: import gzip
  import shutil
  
  def compress_files(input_path, output_path, level):
      with open(input_path, 'rb') as f_in:
          with gzip.open(output_path + '.gz', 'wb', compresslevel=level) as f_out:
              shutil.copyfileobj(f_in, f_out)
      return output_path + '.gz'