Enable strict python bytecompile mode (if rpm supports it)
    - with %_python_bytecompile_errors_terminate_build set to non-zero,
      byte-compilation errors will abort the build, this helps catch out
      silly "improt foo" syntax errors early on
    - not all .py files are valid python (they can be templates, inteded for
      jython consumption etc), and what's valid can depend on the python
      version (notably 2.x vs 3.x) so allow overriding from spec