Show names with hyphens instead of underscores
...on the "/simple" listing.

On the "/simple" listing page, names with hyphens and names with
underscores are currently both pointing to the same set of files due to
the use of `pypiserver.core.normalize_pkgname`. Make a change to display
only names with hyphens on the "/simple" listing page to avoid
duplication.
Remove duplicates from "/simple" index page

The main line repo
(https://github.com/schmir/pypiserver/tree/c1b9386dff6db8f4486d83240d75b6c9c8fa5e1b)
currently lists duplicate entries on the "/simple" index page if the
package name contains hyphens and if both a source file and egg file are
uploaded. One index entry contains hyphens in the name which matches the
source file name and the other entry contains underscores in the name
which matches the egg file. Clicking both links lead to pages which list
the same files (both the source file and the egg file).

The previous fix
(https://github.com/saltycrane/pypiserver/commit/c6966afeeddec52c27e112472b9bf50fdea057ce)
mostly fixed this problem but it did not account for package names that
included an underscore. Change the "/simple" index to use the name from
a non-egg file, since this name should not have hyphens converted to
underscores as egg files do. If there are only egg files, use the egg
file name.
make the /simple page work again when using python 3

this has been broken in the last commit.
add a test for a package that uses both _ and -
move the 'get_prefixes' functionality from simpleindex to own function
make get_prefixes handle packages that use both _ and -

also simplify it a bit