gh-100690: [mock] hide ATTRIB_DENY_LIST and make it immutable (#100819)

Backports: a109454e828ce2d9bde15dea78405f8ffee653ec
Signed-off-by: Chris Withers <chris@simplistix.co.uk>

Author: sobolevn

mock/mock.py

@@ -656,7 +656,7 @@ def __getattr__(self, name):
         elif _is_magic(name):
             raise AttributeError(name)
         if not self._mock_unsafe and (not self._mock_methods or name not in self._mock_methods):
-            if name.startswith(('assert', 'assret', 'asert', 'aseert', 'assrt')) or name in ATTRIB_DENY_LIST:
+            if name.startswith(('assert', 'assret', 'asert', 'aseert', 'assrt')) or name in _ATTRIB_DENY_LIST:
                 raise AttributeError(
                     f"{name!r} is not a valid assertion. Use a spec "
                     f"for the mock if {name!r} is meant to be an attribute.")
@@ -1071,7 +1071,11 @@ def _calls_repr(self, prefix="Calls"):
 
 
 # Denylist for forbidden attribute names in safe mode
-ATTRIB_DENY_LIST = {name.removeprefix("assert_") for name in dir(NonCallableMock) if name.startswith("assert_")}
+_ATTRIB_DENY_LIST = frozenset({
+    name.removeprefix("assert_")
+    for name in dir(NonCallableMock)
+    if name.startswith("assert_")
+})
 
 
 class _AnyComparer(list):