SCons: Drop support for Python 2

We now require SCons 3.0+ (first version with Python 3 support),
and we set min required Python 3 version to 3.5 (3.4 and earlier are
EOL).

platform/SCsub

@@ -1,7 +1,5 @@
 #!/usr/bin/env python
 
-from compat import open_utf8
-
 Import('env')
 
 env.platform_sources = []
@@ -21,7 +19,7 @@ reg_apis += '}\n\n'
 unreg_apis += '}\n'
 
 # NOTE: It is safe to generate this file here, since this is still execute serially
-with open_utf8('register_platform_apis.gen.cpp', 'w') as f:
+with open('register_platform_apis.gen.cpp', 'w', encoding="utf-8") as f:
     f.write(reg_apis_inc)
     f.write(reg_apis)
     f.write(unreg_apis)