py/objfun: Support fun.__globals__ attribute.

This returns a reference to the globals dict associated with the function,
ie the global scope that the function was defined in.  This attribute is
read-only but the dict itself is modifiable, per CPython behaviour.

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/fun_globals.py

@@ -0,0 +1,21 @@
+# test the __globals__ attribute of a function
+
+
+def foo():
+    pass
+
+
+if not hasattr(foo, "__globals__"):
+    print("SKIP")
+    raise SystemExit
+
+print(type(foo.__globals__))
+print(foo.__globals__ is globals())
+
+foo.__globals__["bar"] = 123
+print(bar)
+
+try:
+    foo.__globals__ = None
+except AttributeError:
+    print("AttributeError")