Switch to using pyproject.toml

[blag]

Switch to using pyproject.toml, since that's a nice, declarative way to define a project.

I tried to verify that the same files are included in the distribution by diffing the generated files, here's the output of sdiff between using setup.py and pyproject.toml:

.								.
./signals.py							./signals.py
./migrations							./migrations
./migrations/__init__.py					./migrations/__init__.py
./migrations/0002_auto_20170912_0537.py				./migrations/0002_auto_20170912_0537.py
./migrations/0003_auto_20211015_1406.py				./migrations/0003_auto_20211015_1406.py
./migrations/0001_initial.py					./migrations/0001_initial.py
./models.py							./models.py
./tests								./tests
./tests/__init__.py						./tests/__init__.py
./tests/test_views.py						./tests/test_views.py
./tests/constanats.py						./tests/constanats.py
./tests/test_sd.py						./tests/test_sd.py
./admin								./admin
./admin/__init__.py						./admin/__init__.py
./admin/forms.py						./admin/forms.py
./admin/admin.py						./admin/admin.py
./__init__.py							./__init__.py
./test_softdelete_app						./test_softdelete_app
./test_softdelete_app/models.py					./test_softdelete_app/models.py
./test_softdelete_app/__init__.py				./test_softdelete_app/__init__.py
./test_softdelete_app/exceptions.py				./test_softdelete_app/exceptions.py
./forms.py							./forms.py
./settings.py							./settings.py
./templates							./templates
./templates/base.html						./templates/base.html
./templates/admin						./templates/admin
./templates/admin/softdelete					./templates/admin/softdelete
./templates/admin/softdelete/changeset				./templates/admin/softdelete/changeset
./templates/admin/softdelete/changeset/change_form.html		./templates/admin/softdelete/changeset/change_form.html
./templates/admin/softdelete/softdeleterecord			./templates/admin/softdelete/softdeleterecord
./templates/admin/softdelete/softdeleterecord/change_form.htm	./templates/admin/softdelete/softdeleterecord/change_form.htm
./templates/admin/softdelete/change_form.html			./templates/admin/softdelete/change_form.html
./templates/softdelete						./templates/softdelete
./templates/softdelete/stubs					./templates/softdelete/stubs
./templates/softdelete/stubs/changeset_detail.html		./templates/softdelete/stubs/changeset_detail.html
./templates/softdelete/changeset_list.html			./templates/softdelete/changeset_list.html
./templates/softdelete/base.html				./templates/softdelete/base.html
./templates/softdelete/changeset_form.html			./templates/softdelete/changeset_form.html
./templates/softdelete/changeset_detail.html			./templates/softdelete/changeset_detail.html
./south_migrations						./south_migrations
./south_migrations/__init__.py					./south_migrations/__init__.py
./south_migrations/0002_auto__del_recordset__del_unique_recor	./south_migrations/0002_auto__del_recordset__del_unique_recor
./south_migrations/0001_initial.py				./south_migrations/0001_initial.py
./south_migrations/0003_auto__chg_field_softdeleterecord_obje	./south_migrations/0003_auto__chg_field_softdeleterecord_obje
./urls.py							./urls.py
./views.py							./views.py