Final py3.9 deprecation

[dannysepler]

Closes #2689. All the other deprecations have been taken care of here #2698

This also added a few tests (and snuck in a pytest refactor)

[dannysepler]

could alternatively delete this method? it's only used in nltk/test/classify.doctest and that usage feels unnecessary?

[tomaarsen]

I was unaware of this function, but I could definitely see how it has a nice educatory use. Being able to quickly see the useful methods of a class can be quite useful if you're not entirely aware of the codebase, especially considering that NLTK is definitely not as well documented as I'd like.

There are some classes in NLTK for which users definitely might like to get an overview of the methods that it supports. nltk.tree.Tree, nltk.BigramCollocationFinder and nltk.FreqDist come to mind, but upon trying the last one, I get the following exception:

>>> import nltk
>>> nltk.usage(nltk.FreqDist)
FreqDist supports the following operations:
  - self.B()
  - self.N()
  - self.Nr(r, bins=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "[sic]\nltk\util.py", line 53, in usage
    signature = inspect.signature(method)
  File "[sic]\lib\inspect.py", line 3130, in signature    
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
  File "[sic]\lib\inspect.py", line 2879, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
  File "[sic]\lib\inspect.py", line 2334, in _signature_from_callable
    return _signature_from_builtin(sigcls, obj,
  File "[sic]\lib\inspect.py", line 2145, in _signature_from_builtin
    raise ValueError("no signature found for builtin {!r}".format(func))
ValueError: no signature found for builtin <method 'clear' of 'dict' objects>

Potentially this is because FreqDist subclasses collections' Counter, which again subclasses dict. Note that I also got a similar exception before this PR.

As far as I'm concerned, this can still be merged, as the bug existed before this already regardless. We can just open an issue for it then. However, if you feel like it, you can also include a fix for this in this PR already.

[dannysepler]

good catch!! you're totally right, thanks!

turns out that inspect and some builtin functions don't always work out together. i can't really figure out how to get the failing ones working, my best workaround is to skip them?

[iliakur]

Hmm, what about Python's built-in help command? It gives you both methods and attributes as well as their docstrings. I use it extensively with all kinds of codebases.

I completely agree that the docs need some love, but that's a separate issue imo.

[tomaarsen]

It's indeed a separate issue I'd say.

I'd even consider usage failing on some builtin functions to be a separate issue. If you'd like to get this merged, then we can just open an issue saying that usage is broken in some cases. That is, assuming that the bug still exists.

Regarding the tests, it might be a good idea to check if the output .startswith("MyClass supports the following operations:") or something, or whether the output is indeed a string with at least length ~35 or something. It's not much, but then at least we have some tests to verify that it prints a string with "... supports the following operations: ...".

[stevenbird]

Yes, let's just treat that as a separate issue.

[stevenbird]

Thanks @dannysepler, @tomaarsen, @iliakur