Bug#718818: devhelp: Please build-dep on python3, not python3-dev

[Scott Kitterman]

Package: devhelp
Version: 3.8.2-2
Severity: minor

As nearly as I can determine, devhelp contains only pure python3 that does
not need to be compiled, so the headers and related files in the -dev packages
aren't needed.  We key of the build-dep on -dev for our list of packages that
may need to be rebuilt for a python3 transition, so changing the build-dep
will simplify things a bit for us the next time around.  Additionaly, it will
reduce the build time for devhelp somewhat since fewer packages need to be
installed.