Uncomment the ! assertion.

Co-Authored-By: Nicholas Bollweg <[e-mail]>

IPython/core/inputtransformer2.py

@@ -253,7 +253,7 b' class SystemAssign(TokenTransformBase):'
         lhs = lines[start_line][:start_col]
         end_line = find_end_of_continued_line(lines, start_line)
         rhs = assemble_continued_line(lines, (start_line, start_col), end_line)
-        # assert rhs.startswith('!'), rhs
+        assert rhs.startswith('!'), rhs
         cmd = rhs[1:]
 
         lines_before = lines[:start_line]