Implement GenericMap.__iter__ in C++.

Having GenericMap::keys() be a view makes it easy to iterate from C++,
which the style guide recommends.

python/lsst/afw/typehandling/_GenericMap.cc

@@ -123,7 +123,9 @@ void declareGenericMap(utils::python::WrapperCollection& wrappers, std::string c
                 // Prevent segfaults when assigning a key<Storable> to Python variable, then deleting from map
                 // No existing code depends on being able to modify an item stored by value
                 "key"_a, "default"_a = py::none(), py::return_value_policy::copy);
-        // __iter__ easier to implement in Python
+        cls.def("__iter__",
+                [](Class const& self) { return py::make_iterator(self.keys().begin(), self.keys().end()); },
+                py::keep_alive<0, 1>());
         cls.def("__len__", &Class::size);
         cls.def("__bool__", [](Class const& self) { return !self.empty(); });
         // Private to let a proper view be defined in Python

python/lsst/afw/typehandling/_GenericMap.py

@@ -34,10 +34,6 @@ def __repr__(self):
         className = type(self).__name__
         return className + "({" + ", ".join("%r: %r" % (key, value) for key, value in self.items()) + "})"
 
-    def __iter__(self):
-        for key in self._keys():
-            yield key
-
     # Support equality with any Mapping, including dict
     # Not clear why Mapping.__eq__ doesn't work
     def __eq__(self, other):