SCons: Increase min Python version to 3.6

Current SCons 4.2.0 still supports Python 3.5 but deprecated it, and support
will be removed in the next release.

It's also become needlessly restrictive to prevent ourselves from using Python
3.6 f-Strings, so it's time to up the requirement.

SConstruct

@@ -1,7 +1,7 @@
 #!/usr/bin/env python
 
 EnsureSConsVersion(3, 0, 0)
-EnsurePythonVersion(3, 5)
+EnsurePythonVersion(3, 6)
 
 # System
 import atexit