[PATCH 2 of 2] py3: convert unicode paths given for hgweb config

[Ludovic Chabant]

> We'll probably need to fix the caller to not pass in a unicode. Can
> you copy-paste the traceback?

The caller is my own code. It's basically a custom version of the
hgweb.wsgi file (see
https://www.mercurial-scm.org/repo/hg/file/tip/contrib/hgweb.wsgi).

If we want to be consistent with other things like the hgclient API,
maybe we shouldn't be nice and encode the string for the caller, and
instead reject anything that's not bytes?