fix: refactor code so most things work without sqlalchemy

also refactor tests to help ensure that remains true

Add wutta wrapper for `short_session()`

and several related tweaks for tests, docs

Move tests out of the installed package set

this seems to be better practice. from what i can tell this is a
clean move with no breakage.. and built package files no longer seem
to include test modules

Make 'tests' a proper subpackage again; add some tests

Trying to make more of an effort here...

Overhaul config system, finally replacing edbob (yay!).

* No more `edbob.init()`, now we use `rattail.config.make_config()` instead.
* Adds new `ConfigExtension` mechanism to replace old 'init' config magic.
* Adds config extension for `rattail.db`, to load all Rattail db engines.
* All application entry points have been updated to use `make_config()`.

This also refactors `rattail.db` somewhat in general, to allow for safely
importing it without getting SA-related import errors.

Move some config-related functions to `rattail.db.config`.

This also ensures that `rattail.db.config` may be safely imported, even if
SQLAlchemy isn't available.

General header / Unicode cleanup.

This hopefully updates all module headers such that the file encoding is
utf-8, and uses Unicode literals, etc.

Accept config section name within `db.util.get_engines()`.

This is to set the stage for leveraging this function to establish database
connections to other systems, e.g. for `rattail_locsms`.