Fixes Mypyc #1192

Fix a reference leak in mypyc-generated code for bytes + bytes.

The bytes + bytes primitive for CPyBytes_Concat was marked as stealing the
left operand with steals=[True, False]. However, CPyBytes_Concat does not
steal or decref either argument; it allocates and returns a new bytes object.

Because of this mismatch, the refcount pass skipped emitting a dec_ref for
owned left operands. For code such as:

return (1).to_bytes(4, "big") + (2).to_bytes(4, "big")

the generated IR decrefed only the right operand after CPyBytes_Concat, leaving
the owned left operand alive. Chained concatenations amplified the leak because
intermediate concat results could also become leaked left operands.

This PR removes the incorrect steals=[True, False] annotation from the
bytes + bytes primitive, so both operands are treated as non-stolen and owned
operands are decrefed normally after the call.

The tests verify that the generated IR decrefs both owned operands after
CPyBytes_Concat.