Web.db

* **Updated minimum Python version.** Marrow Package now requires Python 3.6 or later.

* **Removed Python 2 support and version specific code.** The project has been updated to modern Python packaging standards, including modern namespace use. Modern namespaces are wholly incompatible with the previous namespacing mechanism; this project can not be simultaneously installed with any Marrow project that is Python 2 compatible.

**Bugfix release.**
- Updated the `README` and metaproject layout to current Marrow standards.
- Removed extraneous imports and slots where unhelpful or causing issues, such as in the SQLAlchemy adapter. (Thanks `bmillham`!)
- Migrated `MongoDBConnection` from [marrow.mongo](https://github.com/marrow/mongo).

- Initial extract of extension from WebCore.
- SQLAlchemy support.
- Python DB API 2.0 support.
- Experimental MongoEngine support.