devel/nmweb: read mail files in binary mode.

"ju" reported on IRC that browsing

    https://nmbug.notmuchmail.org/nmweb/show/20160719094205.qmf5sjnja6crt5t3%40gotlib

crashed. The underlying issue is that python3 defaults to utf8
decoding files unless they are opened in binary mode. The file in
question (in the nmbug archive; it depends a bit on the routing the
message took) has

        Content-Type: text/plain; charset=iso-8859-1
        Content-Transfer-Encoding: 8bit

and some of it is not valid utf8.