Fixing setting this in Python when using __slots__

Don't attempt to use the class's __dict__ for setting 'this' when
a user has extended a class with:
    __slots__ = ['this'].
Was segfaulting. Now we fall back to a simple PyObject_SetAttr if the
usual approach to setting 'this' in __dict__ does not work.

Closes #1673 Closes #1674

Examples/test-suite/python/python_append_runme.py

@@ -21,3 +21,16 @@ if grabstaticpath() != None:
 Test.static_func()
 if grabstaticpath() != os.path.basename(mypath):
     raise RuntimeError("grabstaticpath failed")
+
+# slots test
+fs = ForSlots()
+if fs.ValidVariable != 99:
+    raise RuntimeError("ValidVariable failed")
+fs.ValidVariable = 11
+if fs.ValidVariable != 11:
+    raise RuntimeError("ValidVariable failed")
+try:
+    fs.Invalid = 22
+    raise RuntimeError("It should not be possible to set a random variable name")
+except AttributeError:
+    pass