GitHub's CODEOWNERS to automate pull request review assignment

Hi all,

I recently learnt of another GitHub feature which may be
of use for Biopython:

https://github.com/blog/2392-introducing-code-owners

The Galaxy Project are now trying this for their repository of
training materials (which is how I heard about the idea):

https://github.com/galaxyproject/training-material/pull/539

The idea is we can label a default owner for files or entire
folders, and if a pull request changes those files, GitHub
will automatically assign their owner as a reviewer.

For example, we might include the following mappings:

Bio/Cluster/* @mdehoon
Bio/SeqIO/* @peterjc
Bio/PopGen/* @tiagoantao
Bio/Phylo/* @etal
...

This can be combined with the branch protection options,
which I proposed using recently:

http://mailman.open-bio.org/pipermail/biopython-dev/2017-August/021861.html

Peter