code: def build_dict_to_sign(self, request, signature_headers):
          
          d = {}
          for header in signature_headers:
              if header == '(request-target)':
                  continue
              d[header] = request.META.get(self.header_canonical(header))
          return d
docs: Build a dict with headers and values used in the signature.
  
          "signature_headers" is a list of lowercase header names.