port svgcolor_lib generation script to Python

port svgcolor_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. Though it
also conveniently side steps some line ending portability issues that were being
worked around with `AWK_OPTIONS`. In Python, this is unnecessary because reading
a file in text mode accounts for both flavors of line endings.

Gitlab: #2118