Fix 'python -m snappy -d' (cannot unpack non-iterable function object)

[cristianchiriac]

Fixes #153.

Since d882cce, get_decompress_function() returns only the decompress function. __main__.py still unpacks method, read_chunk from it, so every python -m snappy -d ... fails with TypeError: cannot unpack non-iterable function object. This happens with -t framing as well as with the default auto.

The change:

• Use the returned function directly. It is called without start_chunk.
• With -t auto, rewind the input after the format check. Guessing the format reads from the file (the framing check consumes the stream header, and the raw check seeks to the end), so the decompressor should start from the beginning. test_formats.py does the same with compressed_stream.seek(0).

I added TestCommandLine to test_formats.py. It compresses a file with python -m snappy -c, then decompresses it with each CLI format (framing, auto) and compares the result with the original. Without the fix both subtests fail with the error above. With it, pytest test_formats.py test_snappy.py passes (23 tests, Python 3.12).

Note: CI currently runs only test_snappy.py, so this test (and the existing format tests) only run locally unless the workflow is extended. I can add that here if you'd like.

[martindurant]

I believe this is the same as #154