bug#24557: python importer: extend requirements import

[ng0]

One example:

address@hidden ~$ guix import pypi hyperkitty

Starting download of /tmp/guix-file.yrqmcv
>From 
>https://pypi.python.org/packages/47/2d/cc2adf4bc0ed7bb4189b3146dde921c9a6ecc33f0f87c96c744d89bab154/HyperKitty-1.0.3.tar.gz...
 …1.0.3.tar.gz  1.5MiB              1.1MiB/s 00:01 [####################] 100.0%
tar: HyperKitty-1.0.3/requirements.txt: Not found in archive
tar: Exiting with failure status due to previous errors
guix import: warning: 'tar xf' failed with exit code 512
(package
  (name "python-hyperkitty")
  (version "1.0.3")


When you look at the contained setup.py of hyperkitty, it has this
defined in setup.py.

# Requirements REQUIRES = [
etc etc

I've seen this in some variations.

The python importer should be extended to include these not very unusual
cases.
-- 
              ng0