Enable brotli decompression if it is available

[immerrr]

This PR enables brotli decompression of responses if it is available (via brotli, brotlipy or brotlicffi)

I have added all three brotli variations to the test suite, but I'm at a bit of a loss, because httpbin brings in brotlipy unconditionally, so there is no real way to test:

• how vcrpy will behave without any brotli implementation
• how vcrpy will behave with a different brotli implementation (in theory, all three support the same brotli.decompress(data) API)

Happy to hear any feedback, but also feel free to make any adjustments that feel right (I have enabled edits from maintainers).

Fixes #599

[immerrr]

@jairhenrique would you be able to look at this at your convenience?

[CharString]

Compression is an ongoing field of research. This suggestion will make it easier to adopt new algorithms.

[CharString]

It is easier to expand with future or custom decompressors if AVAILABLE_DECOMPRESSORS is a Dict[[str], Callable[[bytes], str]]

AVAILABLE_DECOMPRESSORS = {
    "brotli": brotli.decompress,
    "deflate": zlib.decompress,
    "gzip": lambda body: zlib.decompress(body, zlib.MAX_WBITS | 16),
}

Then decompress_body can just be

def decompress_body(body, encoding):
    return AVAILABLE_DECOMPRESSORS[encoding](body)

Adding a new scheme will be as easy as adding a function to the dict.

[finswimmer]

Hey,

what's the reason that this hasn't been merged until now? 🤔

fin swimmer