Anyio compatability

[dolamroth]

Starlette itself is anyio-compatible. However, underlying implementations of low-level libraries are mostly not.

• With current scheme, SQLAlchemy is main core part, which is not anyio-compatible.
• Anyio-compatible websockets are available, if using hypercorn (beta, as of right now) + anysocks/asyncwebsockets.
• There is no DB engines (at least for Postgres) with anyio, but there are separate implementations for asyncio and trio
• Same for redis / mqtt / rmq
• ~There is yet no implementation of SMTP on trio / anyio~ https://pypi.org/project/smtpproto/

[dolamroth]

The framework itself has been adapted to anyio. However, asyncio-only requirements remain.