deprecate: deprecate get and set node methods

[12mohaned]

This pull request deprecates _get_node() and _set_node() methods, it was similar to this PR, but I forgot to amend the required changes instead, I accidentally removed the methods. Sorry for the inconvenience @stevenbird .

[tomaarsen]

NLTK has implemented a useful decorator for cases like these. I recommend that we use it. See an example below:

nltk/nltk/metrics/agreement.py

Lines 164 to 178 in edf47c0

	@deprecated("Use Nk, Nik or Nck instead")
	def N(self, k=None, i=None, c=None):
	"""Implements the "n-notation" used in Artstein and Poesio (2007)"""
	if k is not None and i is None and c is None:
	ret = self.Nk(k)
	elif k is not None and i is not None and c is None:
	ret = self.Nik(i, k)
	elif k is not None and c is not None and i is None:
	ret = self.Nck(c, k)
	else:
	raise ValueError(
	f"You must pass either i or c, not both! (k={k!r},i={i!r},c={c!r})"
	)
	log.debug("Count on N[%s,%s,%s]: %d", k, i, c, ret)
	return ret

i.e. simply adding @deprecated("Use set_label() instead") before both of the methods. The body can become pass or kept how it was. See the code for deprecated here.

[tomaarsen]

This is definitely preferable, however the _get_node() is deprecated section of the deprecation message is unnecessary, as @deprecated already adds this. Right now, the output is:

>>> from nltk.tree import Tree
>>> t = Tree.fromstring('(S (NP (D the) (N dog)) (VP (V chased) (NP (D the) (N cat))))')
>>> t._get_node()  
<stdin>:1: DeprecationWarning: 
  Function _get_node() has been deprecated.  _get_node() is
  deprecated, use label() instead

[tomaarsen]

Looks good to me

[tomaarsen]

@12mohaned
#2903 has fixed a broken test case which snuck into this PR. I've merged develop into this PR to resolve this broken test.