port brewer_lib generation script to Python

port brewer_lib generation script to Python

Note that this does not introduce an extra build dependency in any of the three
build systems:

  1. Autotools: these steps are done during construction of the portable source
     tarball, during which Python 3 is already required (see autogen.sh).

  2. CMake: Python 3 is already required by and used in the top level
     CMakeLists.txt.

  3. MS Build: Python 3 is already used in version generation in
     lib/version/version.vcxproj.

This change is motivated by the goal of removing a dependency on Awk.

Gitlab: #2118