Add Boost::Python to Python modules link libraries

[JenusL]

Small patch that adds the missing Boost linking to the python modules PyIex and PyImath
This closes #573 and closes #594

[meshula]

This seems fine, @antonio-rojas, can you verify whether this fixes your issue?

[JenusL]

I'm building on Windows 10 with CMake 3.15.4, Visual Studio 2019, MSVC 14.00.
First I open a dev console: "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\Tools\VsDevCmd.bat" -arch=amd64 -host_arch=amd64 -winsdk=10.0.18362.0 -vcvars_ver=14.0
Then I generate with cmake -G "Visual Studio 16 2019" -T v140 and then build.

Without the patch, the generated iex_python2.vcxproj and imath_python2.vcxproj wouldn't get boost_python3-vc140-mt-x64-1_70.lib added to additional dependencies.

[xlietz]

@meshula I think you may have noticed already, our CI isn't finding boost or boost python on Windows, still on my to-do list. The CI still doesn't find it with this change, but if it works for @JenusL that's a good start. @kdt3rd ?

[JenusL]

I had a look at the CI logs for this PR and when Boost is being built it sets BOOST_ROOT = c:\Program Files\Boost\1.66.0
But then in the OpenEXR build I can see this in the log:

Environment variable Boost_ROOT is set to:

    C:\Program Files\Boost\1.69.0

Boost 1.69.0 is never being built and I don't know where it got that version number from. But I would start the investigation there.