Readd error for backwards compatibility

Author: rhpvorderman

CHANGELOG.rst

@@ -31,7 +31,6 @@ that PyPy is no longer supported.
   faster.
 + igzip.decompress has 30% less overhead when called.
 + ``isal_zlib`` functions now raise ``isal_zlib.error`` on error.
-  ``isal_zlib.IsalError`` has been removed.
 + The base class for ``isal_zlib.error`` and ``igzip_lib.IsalError`` is now
   ``Exception`` instead of ``OSError``.
 + GzipReader now uses larger input and output buffers (128k) by default and

src/isal/isal_zlib.c

@@ -1190,6 +1190,11 @@ PyInit_isal_zlib(void)
         return NULL;
     }
 
+    Py_INCREF(IsalError);
+    if (PyModule_AddObject(m, "IsalError", IsalError) < 0) {
+        return NULL;
+    }
+
     PyTypeObject *Comptype = (PyTypeObject *)&IsalZlibCompType;
     if (PyType_Ready(Comptype) != 0) { 
         return NULL;