Switch to httpx for HTTP2 support

[marcelveldt]

We should use HTTP/2 to connect to the Hue bridge:

• Evenstream has keepalive builtin so we can remove the workaround
• No more request throttling needed

aiohttp however does not yet support http2 so we should move to httpx

[marcelveldt]

No more need for workaround to detect if the connection is still alive if we switch to HTTP2

[marcelveldt]

[Bizarrus]

Hue briges accepts keep-alive, but wont handle it.

The bridge will not send automatically heartbeats or pings.