py/objtype: Implement __delattr__ and __setattr__.

This patch implements support for class methods __delattr__ and __setattr__
for customising attribute access.  It is controlled by the config option
MICROPY_PY_DELATTR_SETATTR and is disabled by default.

tests/basics/class_delattr_setattr.py

@@ -0,0 +1,63 @@
+# test __delattr__ and __setattr__
+
+# feature test for __setattr__/__delattr__
+try:
+    class Test():
+        def __delattr__(self, attr): pass
+    del Test().noexist
+except AttributeError:
+    import sys
+    print('SKIP')
+    sys.exit()
+
+# this class just prints the calls to see if they were executed
+class A():
+    def __getattr__(self, attr):
+        print('get', attr)
+        return 1
+    def __setattr__(self, attr, val):
+        print('set', attr, val)
+    def __delattr__(self, attr):
+        print('del', attr)
+a = A()
+
+# check basic behaviour
+print(getattr(a, 'foo'))
+setattr(a, 'bar', 2)
+delattr(a, 'baz')
+
+# check meta behaviour
+getattr(a, '__getattr__') # should not call A.__getattr__
+getattr(a, '__setattr__') # should not call A.__getattr__
+getattr(a, '__delattr__') # should not call A.__getattr__
+setattr(a, '__setattr__', 1) # should call A.__setattr__
+delattr(a, '__delattr__') # should call A.__delattr__
+
+# this class acts like a dictionary
+class B:
+    def __init__(self, d):
+        # store the dict in the class, not instance, so
+        # we don't get infinite recursion in __getattr_
+        B.d = d
+
+    def __getattr__(self, attr):
+        if attr in B.d:
+            return B.d[attr]
+        else:
+            raise AttributeError(attr)
+
+    def __setattr__(self, attr, value):
+        B.d[attr] = value
+
+    def __delattr__(self, attr):
+        del B.d[attr]
+
+a = B({"a":1, "b":2})
+print(a.a, a.b)
+a.a = 3
+print(a.a, a.b)
+del a.a
+try:
+    print(a.a)
+except AttributeError:
+    print("AttributeError")