Issue 35942: posixmodule.c:path_converter() returns an invalid exception message for broken PathLike objects

>>> class K:
...   def __fspath__(self):
...     return 1
...
>>> import os
>>> os.stat(K())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: stat: path should be string, bytes, os.PathLike or integer, not int


This error message is internally inconsistent:
- it suggests that the error is about the path argument whereas it's in fact about the value returned from `__fspath__()`
- it hilariously states "should be integer, not int"
- it claims os.PathLike is fine as a return value from `__fspath__()` whereas it's not


I would advise removing the custom `__fspath__()` handling from `path_converter` and just directly using PyOS_FSPath which returns a valid error in this case (example from pypy3):

>>>> class K:
....   def __fspath__(self):
....     return 1
....
>>>> import os
>>>> os.open(K(), os.O_RDONLY)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: expected K.__fspath__() to return str or bytes, not int

The custom `__fspath__()` handling is used in `path_converter` for better error reporting. The error message can include names of the function and the argument. PyOS_FSPath() can not provides this information.

I agree that error reporting about incorrect type of the __fspath__() result should be improved.

New changeset 09fbcd6085e18b534fd4161844ff39f77eb4a082 by Pablo Galindo in branch 'master':
bpo-35942: Improve the error message if __fspath__ returns invalid types in path_converter (GH-11831)
https://github.com/python/cpython/commit/09fbcd6085e18b534fd4161844ff39f77eb4a082

New changeset a01065a3588d3f0d0c57ea35107aa97e722fe2b2 by Miss Islington (bot) in branch '3.7':
bpo-35942: Improve the error message if __fspath__ returns invalid types in path_converter (GH-11831)
https://github.com/python/cpython/commit/a01065a3588d3f0d0c57ea35107aa97e722fe2b2

New changeset 8d01eb49fc7ff914feeb2e2090d1d6ef15c932ab by Serhiy Storchaka in branch 'master':
Fix syntax warnings in tests introduced in bpo-35942. (GH-11934)
https://github.com/python/cpython/commit/8d01eb49fc7ff914feeb2e2090d1d6ef15c932ab

New changeset a8834905df853510f38d4e2d628bed8229fd7482 by Miss Islington (bot) in branch '3.7':
Fix syntax warnings in tests introduced in bpo-35942. (GH-11934)
https://github.com/python/cpython/commit/a8834905df853510f38d4e2d628bed8229fd7482