Test failures with Python 3.13.0b1: AttributeError: module 'unittest' has no attribute 'findTestCases'

[mgorny]

With Python 3.13.0b1:

$ python3.13 test/run_all_tests.py
Patching sys.path to test local version. Testing Version: 3.5
Traceback (most recent call last):                                                                                                     
  File "/tmp/portage/dev-python/pyserial-3.5-r2/work/pyserial-3.5/test/run_all_tests.py", line 40, in <module>
    testsuite = unittest.findTestCases(module)                                                                                         
                ^^^^^^^^^^^^^^^^^^^^^^                                                                                                 
  File "/usr/lib/python3.13/unittest/__init__.py", line 80, in __getattr__
    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")  
AttributeError: module 'unittest' has no attribute 'findTestCases'