Fix ExtSlice crash with Python <= 3.8
BUG: 426292 FIXED-IN: 5.6.1 The dimensions of an ExtSlice node are _slice nodes rather than _expr, so this would crash whenever encountering one. -- This was introduced in commit 05e95cac "Initial support for CPython 3.9". When I replaced `SliceAst` with `ExpressionAst` in the relevant line of python39.sdef, conversionGenerator.py *also* changed `_slice` to `_expr` here (because the type used is implicitly determined in a stupid way) and I failed to notice that difference in the newly-generated code. I couldn't see an easy way to fix the implicit type thing, so this commit just bypasses that mechanism and uses CODE instead.
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