Update brotli support to use the brotlicffi package

[tomchristie]

The brotlipy package has been superseded by the brotlicffi package.

See #1604

[GeekDuanLian]

Hey, please follow the instructions in the brotlicffi document to import:

try:
    import brotlicffi as brotli
except ImportError:
    import brotli

Because it is difficult to install brotlicffi on cygwin, but Brotli is easy (existing software package).
So on cygwin, only can use import brotli.

[GeekDuanLian]

I suggest that the way to install dependencies is also in accordance with the brotlicffi documentation:

install_requires=[
    "brotli; platform_python_implementation == 'CPython'",
    "brotlicffi; platform_python_implementation != 'CPython'"
]