This commit reverts the API change introduced in b3302398bc
that changes `ir.qweb#render()` to return a unicode string instead of a utf8
encoded bytestring.
Returning a unicode string is undoubtedly cleaner and a step in the direction
of python 3 but still is an API change that can break third party modules.
Let's wait the migration to python 3 before breaking other people's toys.