This patch ports the tempurl middleware over to PY3. We use
an "all-native" string model, where we convert WSGI strings
as soon as we see them. It helps to deal with HMAC.
Aaand, we fix formpost along the way. It _clearly_ was
doing a wrong thing, encoding the same body several times
if we had several keys. On py2 it wasn't noticeable, but
on py3 this breaks, because the bytes type cannot be encoded
again.
Change-Id: I69974cc8a39731c980b54137b799a36b2e63a44a
9f1ef35630