python27Packages.application: 2.7.0 -> 2.8.0 and fix build

The build is currently broken due to failure to build `darcs` to fetch the src
package. The homepage is already their GitHub repo, and it appears to be the
active src of development anyways. See #83718

I came across this while debugging this failure:
https://hydra.nixos.org/build/115510612

Note that the `application` dependency *does* succeed on Hydra, because it's
already on local disk in Hydra's store, but I cannot rebuild locally because it
has prefer local builds.
https://hydra.nixos.org/build/115512559

This package is not reproducible on 20.03 or buildable outside of Hydra, so I
intend to backport the fix.

CC @NixOS/nixos-release-managers

ZHF: #80379