fix: add startup workaround for trainwreck query bug

not at all clear why, but c621e21cd4b4a2a408d8a32ace0b34afe6fdf526 has
introduced some strange bug where the "first" trainwreck query will
raise an error and yet subsequent queries work fine.

so now in the startup sequence we simply force a query, which for
unknown reasons does *not* raise any error, and will somehow magically
prevent the error when a "normal" trainwreck query happens later.

AFAIK this is a "benign" operation and causes no real side effects
other than preventing the strange error. fingers crossed, this is
fine and will not cause further problems...??!

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.

General header / Unicode cleanup.

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