fix eden build with python 3.12

Summary:
X-link: facebookincubator/velox#7653

fix eden build with python 3.12

python 3.12 removes many long deprecated unittest features including  _TextTestResult (deprecated in Python 3.2)

notices as fedora 39 ships with python 3.12

X-link: facebook/sapling#778

Differential Revision: D51455615

fbshipit-source-id: f690f14b24b2645578113f769669179fba476657

build/fbcode_builder/CMake/fb_py_test_main.py

@@ -194,7 +194,7 @@ def fileno(self):
         return self._fileno
 
 
-class BuckTestResult(unittest._TextTestResult):
+class BuckTestResult(unittest.TextTestResult):
     """
     Our own TestResult class that outputs data in a format that can be easily
     parsed by buck's test runner.