cowbuilder/pbuilder: require newer version from experimental

[Ole Streicher <olebole@debian.org>]

Hi,

to build an "experimental" version of one of my packages, I need to
specify a package that is in unstable (1.0.5-1) and in experimental
(1.1~b1-1), and I need the experimental version here.

With "cowbuilder --login --save-after-login", I have put the
"experimental" distribution into /etc/apt/sources.list:

(chroot) # cat /etc/apt/sources.list
deb http://ftp.de.debian.org/debian/ sid main
deb http://ftp.de.debian.org/debian experimental main

and I did "sudo cowbuilder --update" afterwards. When I then do a 
"cowbuilder --login", I can install the needed version manually:

(chroot) # apt-get install python-astropy-helpers=1.1~b1-1
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed: [...]

However, when I now try to use this to build my package, it does not
work:

$ pdebuild
[...]
This package is uninstallable
Dependency is not satisfiable: python-astropy-helpers (>= 1.1~)
[...]

The package I am trying to build is python-astropy, from the alioth git:

http://anonscm.debian.org/cgit/debian-astro/packages/python-astropy.git/tree/?h=experimental

What could be the cause that the dependency is not satisfied from
experimental here?

Best regards

Ole