ObjectProxy: test i* numeric magic methods
ObjectProxy: unwrap multi-proxies of 2nd operand

in __cmp__, __subclasscheck__, and __div__
Fixes test_div_error_proxy with 1st operand being a proxy and 2nd multi-proxy.
ObjectProxy: fix ternary pow operator

by unwrapping other and modulo operands.
Add expected failing tests for ternary operator with non-proxy 1st arg.
Rename private module variable PY3 to _PY3