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- ICPC can't handle the NCVirt trampoline which returns a non-copyable
  type, which is likely due to a constexpr/SFINAE issue. This disables
  the test on that compiler so that at least the rest can be tested.

If operators.h is included, int_ function in the `detail`
namespace will shadow pybind11::int_ type, so the fully qualified
name has to be used.

For example keep_alive<0,1>() should work where the return value may sometimes be None. At present a "Could not allocate weak reference!" exception is thrown.
Update documentation to clarify behaviour of keep_alive when nurse is None or does not support weak references.

This is required since format descriptors for string types that
were using PYBIND11_DESCR were causing problems on C++14 on Linux.

Although this is technically a breaking change, it shouldn't cause
problems since the only use of format strings is passing them to
buffer_info constructor which expects std::string.

Note: for non-structured types, the const char * value is still
accessible via ::value for compatibility purpose.