Obj: refuse to auto-vivify dunder and sunder attributes

[Deln0r]

Closes #176.

Problem

Obj.__getattr__ auto-vivifies missing attributes into empty child Obj instances. That is intentional for the documented keyword-escape pattern (obj.from_ maps to the from key), but it interacts badly with Python introspection tooling:

• inspect.unwrap(obj) probes __wrapped__, gets a fresh Obj back, follows it, gets another fresh Obj, and the underlying Obj now holds a recursive __wrapped entry. Any subsequent serialize() or __repr__() call recurses until RecursionError — exactly the traceback in the issue.
• IPython probes _ipython_canary_method_should_not_exist_ (a sunder name) on every object it displays, leaving the same kind of stray entry.

Fix

Refuse to create attributes whose name starts AND ends with an underscore. The keyword-escape case (single trailing underscore, e.g. from_) is unaffected because the leading underscore is absent. Defined dunder methods (__init__, __repr__, …) are unaffected because __getattr__ is only called when normal attribute lookup has already failed.

def __getattr__(self, name):
    if name.startswith("_") and name.endswith("_"):
        raise AttributeError(name)
    name = name.rstrip("_")
    ...

This is the exact shape proposed by the issue author.

Test plan

A new mautrix/types/util/obj_test.py covers:

Test	Behaviour
test_obj_basic_attribute_access	Normal obj.name = ... access still works
test_obj_trailing_underscore_keyword_escape	obj.from_ still maps to the from key (no regression)
test_obj_dunder_attribute_raises	obj.__wrapped__ raises and does NOT pollute __dict__
test_obj_sunder_attribute_raises	obj._ipython_canary_method_should_not_exist_ raises
test_obj_inspect_unwrap_does_not_recurse	End-to-end: inspect.unwrap(o) is o, o.serialize() == {}

$ pytest mautrix/types/util/obj_test.py -v
5 passed