Revert "pythongh-105063: Disable test_peg_generator.TestCParser bco. ref leaks (python#106024)"

This reverts commit 41ad4df.

Author: StanFromIreland

Lib/test/test_peg_generator/test_c_parser.py

@@ -72,18 +72,8 @@ def test_parse(self):
 @support.requires_subprocess()
 class TestCParser(unittest.TestCase):
 
-    _has_run = False
-
     @classmethod
     def setUpClass(cls):
-        if cls._has_run:
-            # Since gh-104798 (Use setuptools in peg-generator and reenable
-            # tests), this test case has been producing ref leaks. Initial
-            # debugging points to bug(s) in setuptools and/or importlib.
-            # See gh-105063 for more info.
-            raise unittest.SkipTest("gh-105063: can not rerun because of ref. leaks")
-        cls._has_run = True
-
         # When running under regtest, a separate tempdir is used
         # as the current directory and watched for left-overs.
         # Reusing that as the base for temporary directories