update version to 3.0-something

[ctb]

Right now khmer master installs as 2.1.2, which causes problems for dependencies that expect certain internal behavior. Can we update to 3.0-something?

Reading https://www.python.org/dev/peps/pep-0440/#pre-releases, it looks like 3.0a1 is the logical next step. I'll submit a PR next time it bugs me. We should also update the release checklist to make sure we bump the version numbers on master after a release :)

[ctb]

I told versioneer about the new version like so:

git checkout master
git pull
git tag v3.0-a1
git push --tags

If this turns out to be the "right" way then we should update the release doc so that for major increments they're done right after a release. or maybe not, it's not going to be a frequently occurring problem!