MSVC iso646.h conflict in boost/python/operators.hpp

MSVC includes iso646.h under various circumstances, which is easily worked around and is actually seen elsewhere in MPL (Boost issue https://svn.boost.org/trac/boost/ticket/3018). This applies strictly to MSVC, and when iso646.h is included from various means. The trac specifically said use github for Boost.Python related issues, so here I am.

iso646.h defines various bitwise macros which are conflicting with the python operators (or, and, xor).

This is easily fixed with prefix/postfix push_macro and pop_macro's for the appropriate functions.

I have attached a diff against 1.61.

[operators.patch.txt](https://github.com/boostorg/python/files/267087/operators.patch.txt)