Fix 'Chassis' object has no attribute 'get_num_psu' in test_psu.py

[nonodark]

Why I did it

Fix 'Chassis' object has no attribute 'get_num_psu' in test_psu.py

How I did it

Update 'platform_chassis.get_num_psu()' to 'platform_chassis.get_num_psus()'

How to verify it

root@sonic:~# pde-test -v test_psu.py
==================================================================== test session starts ====================================================================
platform linux -- Python 3.9.2, pytest-7.4.4, pluggy-1.3.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /usr/local/sonic_pde_tests
collected 13 items                                                                                                                                          

test_psu.py::test_for_num_psus PASSED                                                                                                                 [  7%]
test_psu.py::test_for_psu_present PASSED                                                                                                              [ 15%]
test_psu.py::test_for_psu_notpresent PASSED                                                                                                           [ 23%]
test_psu.py::test_for_psu_status PASSED                                                                                                               [ 30%]
test_psu.py::test_for_psu_serial_num PASSED                                                                                                           [ 38%]
test_psu.py::test_for_psu_model PASSED                                                                                                                [ 46%]
test_psu.py::test_for_psu_voltage PASSED                                                                                                              [ 53%]
test_psu.py::test_for_psu_current PASSED                                                                                                              [ 61%]
test_psu.py::test_for_psu_power PASSED                                                                                                                [ 69%]
test_psu.py::test_for_psu_input_voltage PASSED                                                                                                        [ 76%]
test_psu.py::test_for_psu_input_current PASSED                                                                                                        [ 84%]
test_psu.py::test_for_psu_capacity PASSED                                                                                                             [ 92%]
test_psu.py::test_for_psu_type PASSED                                                                                                                 [100%]

===================================================================== warnings summary ======================================================================
test_psu.py:3
  /usr/local/sonic_pde_tests/test_psu.py:3: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
    import imp

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=============================================================== 13 passed, 1 warning in 1.37s ===============================================================

[nonodark]

@geans-pin pls help review, thanks

[geans-pin]

Approved

[nonodark]

@geans-pin @lizhuhuams @qiluo-msft please help merge, thanks.