[lldb] Remove uses of six module (NFC)

With lldb (& llvm) requiring Python 3.6+, use of the `six` module can be removed.

Differential Revision: https://reviews.llvm.org/D131304

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -429,7 +429,7 @@ ScriptInterpreterPythonImpl::ScriptInterpreterPythonImpl(Debugger &debugger)
   // Reloading modules requires a different syntax in Python 2 and Python 3.
   // This provides a consistent syntax no matter what version of Python.
   run_string.Clear();
-  run_string.Printf("run_one_line (%s, 'from six.moves import reload_module')",
+  run_string.Printf("run_one_line (%s, 'from importlib import reload as reload_module')",
                     m_dictionary_name.c_str());
   PyRun_SimpleString(run_string.GetData());