As Steve mentioned in the discussion about PEP 570 [1], some changes of parameters to positional-only are breaking (although there is no breaks in the stdlib code). Before making parameters positional-only we should add a deprecation warning for passing them as keyword arguments. Similarly to the code used in the UserDict constructor (see issue22609).

The following PR adds deprecation warnings for other parameters which should be positional-only. It also fixes bugs about nonavailability to pass special keyword names like "self" or "func". Just one example:

>>> import functools
>>> def f(self, func): pass
... 
>>> functools.partialmethod(f, func=chr)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() got multiple values for argument 'func'


[1] https://discuss.python.org/t/pep-570-python-positional-only-parameters/1078/53