Add support for Google's Brotli Python implementation about httpx

[tomchristie]

Is that different to the implementation that we currently support? https://github.com/encode/http3/blob/master/http3/decoders.py#L83

[sethmlarson]

Unfortunately it is: https://github.com/urllib3/urllib3/blob/master/src/urllib3/response.py#L103

[tomchristie]

Gotcha. What’s the difference?

[sethmlarson]

Brotlipy provides decompress()/flush(), Google's Brotli only provides process()

[halbow]

I can take a look at this !

[StephenBrown2]

This has been added in #156

[sethmlarson]

Yep! Closing this issue. Thanks @StephenBrown2