Merge remote-tracking branch 'origin/dev-python/execnet'

Merge remote-tracking branch 'origin/dev-python/execnet'

Pull in this per-package branch with a subtree merge [1]:

  $ git merge -s ours --no-commit origin/dev-python/execnet
  $ git read-tree --prefix=dev-python/dev-python/execnet/ -u origin/dev-python/execnet
  $ git commit -v

so folks who aren't using a version of Layman that supports submodules
(currently everybody) can use this modular package.

[1]: https://www.kernel.org/pub/software/scm/git/docs/howto/using-merge-subtree.html