Here's the function that is decompressing them on the server coordinator.py L291. It's not quite what you need though since it's not reading from a file.
Here's what I use locally based on the above:
with open(config.replay, "rb") as rpfile:
decoder = zstd.ZstdDecompressor()
decoded_data = decoder.decompress(rpfile.read())
replay = json.loads(decoded_data.strip())