test_compression_method_is_applied_to_file_entries built a real zip and
read back compress_type — but that only confirms zipfile.ZipFile()
honors its own compression= kwarg, which is documented stdlib behavior,
not something our code could get wrong. ZipExportSink's only
responsibility is forwarding compression/compresslevel unchanged to
ZipFile(); assert that directly against the mocked constructor call.
test_compressing_method_beats_stored asserted that DEFLATED produces a
smaller archive than STORED — that's zlib's job, not ours.
test_compression_method_is_applied_to_file_entries already covers what
our code is actually responsible for: threading the requested
compression method through to the zip entry's compress_type.
stumpylog