* support nested encodings
* support optional `br` encoding, if brotli package is installed
* support optional 'compress' encoding, if ncompress package is installed
* response `Content-Encoding` has only unprocessed encodings, or removed
* response `Content-Length` is decoded length (usable for filesize metadata)
* use zlib for both deflate and gzip decompression
* some elements taken from yt-dlp: thx especially coletdjnz
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