Drop special cross reference helper for exceptions, functions and methods

[tk0miya]

Is your feature request related to a problem? Please describe.

So far, :py:exc: role automatically searches exception classes under exceptions module. But, since python3, exception classes has been moved into builtins module. So this fallback works fine only for python2 docs.

.. py:exception:: exceptions.Exception

:py:exc:`Exception` can refer `exceptions.Exception` automatically even if no ``py:module::`` given.

In addition, this fallback is a part of python domain, not intersphinx. So it only works for a document contains descriptions for standard exceptions. It does not work for a document only refers python standard exceptions.

Side note: python domain also has another special support for :py:func: and :py:meth:. It also automatically searches functions and methods under object.

.. py:method:: object.__str__()

:py:func:`__str__` and `:py:meth:`__str__` can refer `object.__str__()` automatically even if no ``py:class:`` given.

I don't know why python domain searches them on object.* namespace. I can understand if it searches under builtins module. Anyway, this is also meaningless.

Describe the solution you'd like
Drop these special treatment from python domain.

Describe alternatives you've considered
Nothing.

Additional context
Nothing.

[shimizukawa]

+1 to drop special cross reference helper

[tk0miya]

Note: python-docs declares exceptions classes as root item (not under builtins package):

$ python -m sphinx.ext.intersphinx https://docs.python.org/3/objects.inv
...
py:exception
    ArithmeticError                          library/exceptions.html#ArithmeticError
    AssertionError                           library/exceptions.html#AssertionError
    AttributeError                           library/exceptions.html#AttributeError
    BaseException                            library/exceptions.html#BaseException
    BlockingIOError                          library/exceptions.html#BlockingIOError
    BrokenPipeError                          library/exceptions.html#BrokenPipeError
    BufferError                              library/exceptions.html#BufferError
    BytesWarning                             library/exceptions.html#BytesWarning
    ChildProcessError                        library/exceptions.html#ChildProcessError
    ConnectionAbortedError                   library/exceptions.html#ConnectionAbortedError
    ConnectionError                          library/exceptions.html#ConnectionError
    ConnectionRefusedError                   library/exceptions.html#ConnectionRefusedError
    ConnectionResetError                     library/exceptions.html#ConnectionResetError
    DeprecationWarning                       library/exceptions.html#DeprecationWarning
    EOFError                                 library/exceptions.html#EOFError
    EnvironmentError                         library/exceptions.html#EnvironmentError
    Exception                                library/exceptions.html#Exception
...