feat: upgrade sqlalchemy to 1.4

[kylejb]

Closes #181

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Summary

• [x] Update SQLAlchemy to v1.4.
• [x] Update SQLAlchemy-Utils to 0.38.2 (supports SQLAlchemy v1.4).
• [x] Add asyncpg to dependencies; this is a required driver to work with SQLAlchemy's create_async_driver.
• [x] Update instantiation of session engine with SQLAlchemy's create_async_driver.
• [ ] Update tests to connect to database asynchronously.

Questions

1. Should we explicitly add greenlet as another dependency? It doesn't seem to cause issues in Docker, but may be required in some local contexts.
2. Would you like me to fix tests to work with the synchronous drivers or update all tests to work with asyncio (i.e., ORM's new async driver and asyncpg)?