Here's another thought:

Let's put string_to_bytes and string_from_bytes into the binascii
module, as a2b_percent and b2a_percent, respectively.

Then parse.py would import them as

  from binascii import a2b_percent as percent_decode_as_bytes
  from binascii import b2a_percent as percent_encode_from_bytes

and add two more functions:

  def percent_encode(<string>, encoding="UTF-8", error="strict", plus=False)
  def percent_decode(<string>, encoding="UTF-8", error="strict", plus=False)

and would add backwards-compatible but deprecated functions for quote
and unquote:

  def quote(s):
      warnings.warn("urllib.parse.quote should be replaced by
percent_encode or percent_encode_from_bytes", FutureDeprecationWarning)
      if isinstance(s, str):
          return percent_encode(s)
      else:
          return percent_encode_from_bytes(s)

  def unquote(s):
      warnings.warn("urllib.parse.unquote should be replaced by
percent_decode or percent_decode_to_bytes", FutureDeprecationWarning)
      if isinstance(s, str):
          return percent_decode(s)
      else:
          return percent_decode(str(s, "ASCII", "strict"))