SCons: Bump min version to 3.1.2, test it on CI with one Linux job

The min SCons version had to be bumped as SCons 3.0 before 3.0.3 seems
broken (see #92043), and there's little gain from supporting 3.0.3-3.0.5.

3.1.2 is also the first version to avoid ambiguities between Python 2
and Python 3 usage, so we finally use it as the minimum baseline.

Also test against Python 3.6 which is also our minimum supported version.
This should help prevent regressions whenever we modernize the build scripts.