Add build-system section to pyproject.toml (PEP-517) (djrobstep#176)

According to this document <https://python-poetry.org/docs/pyproject/#poetry-and-pep-517> a `build-system` section is required in `pyproject.toml` file.

Lack of this section is part of the trouble discussed here: <https://discourse.nixos.org/t/preparing-a-nix-flake-for-a-python-program-migra-using-poetry/12030/1>

Fix wrong homepage address in pyproject.toml (djrobstep#175)

The old address <https://migra.djrobstep.com/> gives 522 status. It's probably outdated.  I ran into this problem because the link is used on <https://pypi.org/project/migra/>.

actually bump to 3 this time

first py3 only release

final py2 release

commit lock file

commit lock file

better deferrable handling

fix mv index creation order

fix creation of views dependent on pks