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.