bug#55808: 29.0.50; If a Python class is on the first line, mark-defun does not mark the entire class

[kobarity]

Hello,

I wrote:
> The following steps will mark only the method baz, not the entire
> class Foo.
>
> 1. emacs -Q
> 2. Load the following Python file using M-x find-file
>
> #+begin_src python
> class Foo:
>     def bar():
>         pass
>
>     def baz():
>         pass
> #+end_src
>
> 3. M-x mark-defun
>
> This issue happens only if the class is on the first line of the file.

It turned out that there was a bug in `python-nav-beginning-of-defun'.
Fixing the bug #56105 also resolved this issue #55808. So #55808 can
now be closed.

Thanks,