Warn if "observe" decorator applied to method with only "self"

[mdickinson]

With the new 'observe' decorator, there's a potentially late-discovered bug that I've seen now in multiple different projects, and that's to forget to add the "event" argument on a previously argument-less on_trait_change target.

It may be worth warning if the target of the "observe" decorator does not have exactly one positional argument in addition to "self".

[kitchoi]

One thing to watch out is that the inspection on callable arguments adds limitations on what kind of callables can be used.
The fact that on_trait_change does signature inspection means a whole bunch of callables cannot be used:

>>> from traits.api import *
>>> class Foo(HasStrictTraits):
...     name = Str()
... 
>>> foo = Foo()
>>> events = []
>>> foo.on_trait_change(events.append, "name")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traits/has_traits.py", line 2557, in on_trait_change
    self._on_trait_change(
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traits/has_traits.py", line 2249, in _on_trait_change
    wrapper = self.wrappers[dispatch](handler, notifiers, target)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traits/trait_notifiers.py", line 412, in __init__
    self.init(handler, owner, target)
  File "/Users/kchoi/Work/ETS/py39-venv/traits-env/lib/python3.9/site-packages/traits/trait_notifiers.py", line 447, in init
    arg_count = handler.__code__.co_argcount
AttributeError: 'builtin_function_or_method' object has no attribute '__code__'

Currently this works:

>>> foo.observe(events.append, "name")
>>> foo.observe(print, "name")

Calling inspect.signature on certain built-in functions will also fail. If one detects if a function is an instance method first then subsequent signature inspection should be okay.

[mdickinson]

Yes: I think it would be a warning, not an error, and we'd only issue the warning in the case that we could inspect the code - if we couldn't inspect the code we'd just let it pass.

[mdickinson]

Having implemented a solution, I'm having second thoughts about this; it's a bit inefficient to be doing runtime type-checking for this. A static type-check may be a better solution. If we get the signature of observe right, presumably mypy could do such a static check for us.

[mdickinson]

Third thoughts: I think this is fine for the observe decorator factory, which is where I've seen the issue most often, but would hesitate to extend the check to uses of the observe method. PR #1529 only solves this for the decorator.