tests/cpydiff: Add diff for overriding __init__.

This adds a CPython diff that explains why calling `super().__init__()` is
required in MicroPython when subclassing a native type (because `__new__`
and `__init__` are not separate functions).
Signed-off-by: David Lechner <david@pybricks.com>

tests/cpydiff/core_class_super_init.py

+"""
+categories: Core,Classes
+description: When inheriting native types, calling a method in ``__init__(self, ...)`` before ``super().__init__()`` raises an ``AttributeError`` (or segfaults if ``MICROPY_BUILTIN_METHOD_CHECK_SELF_ARG`` is not enabled).
+cause: MicroPython does not have separate ``__new__`` and ``__init__`` methods in native types.
+workaround: Call ``super().__init__()`` first.
+"""
+
+
+class L1(list):
+    def __init__(self, a):
+        self.append(a)
+
+
+try:
+    L1(1)
+    print("OK")
+except AttributeError:
+    print("AttributeError")
+
+
+class L2(list):
+    def __init__(self, a):
+        super().__init__()
+        self.append(a)
+
+
+try:
+    L2(1)
+    print("OK")
+except AttributeError:
+    print("AttributeError")