[select] Mark select.poll as @final

[sobolevn]

See:

>>> import select
>>> t = type(select.poll())
>>> t
<class 'select.poll'>
>>> class a(t): ...
... 
Traceback (most recent call last):
  File "<python-input-11>", line 1, in <module>
    class a(t): ...
TypeError: type 'select.poll' is not an acceptable base type

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas (https://github.com/pandas-dev/pandas)
- pandas/core/computation/ops.py:328: error: Need type annotation for "_binary_ops_dict" (hint: "_binary_ops_dict: dict[<type>, <type>] = ...")  [var-annotated]