SqlAlchemy 2.0 support#330

This is a just a stopgap, but I've attempted to add support for SqlAlchemy 2.0. Most `select()` statements were able to be updated to 2.0 syntax without issue, while some would only work with `orm.Query()`. `text()` was added to any query literals. It seems that basic functionality is g2g, but there's some weirdness with the unit tests. The vast majority of top level tests succeed when running the entire suite, but many of the nested tests (relationships, plugins) failed unless they're run on their own. There are also some tests (test_savepoints.py) that either I'm not understanding the intent of or are just broken. Also, I skipped the flask tests, as I'm unfamiliar with the `_SessionSignalEvents` that's being imported in the test file, but it seems to have been removed from Flask-SQLAlchemy. I'm not sure how to adapt the tests to the current state of the flask plugin. I'm not sure what the state of tests were before this fork, and how many were regularly failing, so any context there would be greatly appreciated. I should note that I've only run the tests with sqlite, so the issues I'm running into may be specific that database.