initialize_generic should use repr rather than attr("__repr__")

[asadchev]

in pybind11.h, in initialize_generic the relevant line should be
a.descr = strdup(repr(a.value).cast<std::string>().c_str());
otherwise error is thrown with for example type objects
ImportError: TypeError: descriptor '__repr__' of 'float' object needs an argument

[bstaletic]

This is a duplicate of #2028

[YannickJadoul]

This one was first, though, so thanks for reporting, @asadchev! :-)