bpo-31602: Fix an assertion failure in zipimporter.get_source() in case of a bad zlib.decompress() (GH-3784)

While a rare potential failure (it requires swapping out zlib.decompress() itself and forcing it to return a non-bytes object), this change prevents a potential C-level assertion failure and instead substitutes it with an exception.

Thanks to Oren Milman for the patch.