Specified Boost.Python not found on Boost versions < 1.67

[JenusL]

Boost changed how the python files was named in Boost 1.67. This affects how we search for the Boost.Python component with find_package in CMake.

Boost < 1.67
Python 2.7 name: python
Python 3.6 name: python3

Boost >= 1.67
Python 2.7 name: python27
Python 3.6 name: python36

Current CMakeList is looking for python like this:
find_package(Boost COMPONENTS python${OPENEXR_PYTHON_MAJOR}${OPENEXR_PYTHON_MINOR})
This only works on Boost >= 1.67 and will always fall back to default with earlier boost versions.

I already have a fix for this in my branch and will create a PR.

[cary-ilm]

PR is merged, so closing, thanks.