Support aioquic 1.x and update README.rst regarding HTTP/3

[jlaine]

No code changes are required to use aioquic 1.x. As aioquic now uses semantic versioning, allow any version less than 2.0. This allows users to benefit from improvements and security fixes which have landed in the 1.x releases, see:

https://aioquic.readthedocs.io/en/stable/changelog.html

[jlaine]

Hello Philip!

AFAICT the CI failure is not related to my changes.

As there doesn't seem to be a test suite for the h3 code, I used hypercorn to serve aioquic's demo.py ASGI app and performed requests using the http3_client.py client.

Some additional notes:

• I did not tighten the versioned dependency to require aioquic >= 1.0.0 but it might give you a more manageable set of versions to support.
• At some point it would be good to use QuicPacketType.INITIAL (added in 1.2.0) instead of PACKET_TYPE_INITIAL, but we (@rthalley and I) will be keeping backwards compatibility until 2.0.