Fix test_gast_with_compatibility.py due to the problem of gast in python3.8 (#27433)

test=develop

python/paddle/fluid/tests/unittests/test_gast_with_compatibility.py

@@ -16,6 +16,7 @@ from __future__ import print_function
 
 import ast
 import gast
+import sys
 import textwrap
 import unittest
 
@@ -143,6 +144,19 @@ class TestPythonCompatibility(unittest.TestCase):
         """
         self._check_compatibility(source, target)
 
+    # The 0.3.3 version of gast has a bug in python3.8 that
+    # would cause the following tests to fail. But this 
+    # problem doesn't affect the use of Paddle's related 
+    # functions, therefore, the following tests would be 
+    # disable in python3.8.
+    #
+    # This problem had been fixed and updated to version 
+    # 0.4.1 of gast.
+    #
+    # More information please refer to:
+    # https://github.com/serge-sans-paille/gast/issues/49
+    if sys.version_info < (3, 8):
+
         def test_with(self):
             """
             The fileds `context_expr/optional_vars` of `ast.With` in PY2