Migrate from brotlipy to brotlicffi

[johnthagen]

brotlipy has not seen updates for 4 years: https://pypi.org/project/brotlipy/

It looks like the latest work is published under brotlicffi: https://github.com/python-hyper/brotlicffi

• https://pypi.org/project/brotlicffi/

https://github.com/postmanlabs/httpbin/blob/f8ec666b4d1b654e4ff6aedd356f510dcac09f83/setup.py#L38

[mbra]

Also, the old brotlipy module clashes with https://pypi.org/project/Brotli/ from https://github.com/google/brotli and even though I know such clashes are hard to prevent, not relying on very outdated packages would make this a non-issue.