Even if a Sphinx .inv inventory file is labelled "The remainder of this file...

Even if a Sphinx .inv inventory file is labelled "The remainder of this file is compressed using zlib.", it might not actually be. In this case, don't traceback and simply return the original content. (Closes: #299)

diffoscope/comparators/sphinx.py

@@ -55,6 +55,13 @@ def describe_inventory(filename):
                 tail += line
 
     result = head + b"\n" if head else b""
-    result += zlib.decompress(tail)
 
-    return result.decode("utf-8")
+    try:
+        result += zlib.decompress(tail)
+    except zlib.error:
+        # Even if the file is labelled "The remainder of this file is
+        # compressed using zlib.", it might not actually be. In this case,
+        # simply return the original content.
+        result += tail
+
+    return result.decode("utf-8", "ignore")