pybindings: Mark CellInfo::bel as readonly
pybindings: Mark CellInfo::bel as readonly

bel bindings should be updated with bindBel/unbindBel during placement, or setting the BEL attribute for constraints before placement.

Fixes #522

Signed-off-by: gatecat <c690b4516f836771c8bd4e23cd8f1cebc83093a6@ds0.me>

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