This commit:
1. Formats all of our existing code using `black`.
2. Adds a note about using `black` to `CONTRIBUTING.rst`.
3. Runs `black --check` as part of CI (failing builds that aren't properly formatted).
No change required to upgrade to multiple databases. Aliases are used
to describe the database and these can be manually registered or fall
through to a default alias using connect.
Made get_connection and get_db first class members of the connection class.
Old style _get_connection and _get_db still supported.
Refs: #84#87#93#215
