Refactor test suite and utilities to be more self-contained and better-behaved under Python 3

[bourke]

The test suite has a number of structural issues, e.g. including functionality in the package __init__.py, as well as implicit relative imports left over from Python 2.

[sqla-tester]

Ram Rachum has proposed a fix for this issue in the main branch:

Fix exception causes in lookup.py https://gerrit.sqlalchemy.org/c/sqlalchemy/mako/+/3337