ISO646 header interferes with Boost.Python's logical operator definitions on Windows 10

[dzhoshkun]

This seems to be a known issue:

• MSVC iso646.h conflict in boost/python/operators.hpp boostorg/python#67 (comment)
• build python binding error on windows 10,msvc14,python3.6. boostorg/python#212

The following does not seem to work: https://github.com/PixarAnimationStudios/OpenSubdiv/pull/817/files

[dzhoshkun]

One workaround for #69 would be to refactor all new-style logical op calls to old-style ones and then to exclude the ciso646 header.