python: Better use iterators

In Python 2, iterators had a .next() method.

In Python 3, instead they have a .__next__() method, which is
automatically called by the next() builtin.

In addition, it is better to use the iter() builtin to create an
iterator, rather than calling its __iter__() method.

These were also introduced in Python 2.6, so using it makes the script
compatible with Python 2 and 3.

Signed-off-by: Mathieu Bridon <bochecha@daitauha.fr>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

src/compiler/glsl/ir_expression_operation.py

@@ -62,7 +62,7 @@ class type_signature_iter(object):
    def __iter__(self):
       return self
 
-   def next(self):
+   def __next__(self):
       if self.i < len(self.source_types):
          i = self.i
          self.i += 1
@@ -76,6 +76,8 @@ class type_signature_iter(object):
       else:
          raise StopIteration()
 
+   next = __next__
+
 
 uint_type = type("unsigned", "u", "GLSL_TYPE_UINT")
 int_type = type("int", "i", "GLSL_TYPE_INT")