gh-137716: Fix double period in AttributeError message for invalid mock assertions (#137717)

Lib/unittest/mock.py

@@ -700,7 +700,7 @@ class NonCallableMock(Base):
             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.")
+                    f"for the mock if {name!r} is meant to be an attribute")
 
         with NonCallableMock._lock:
             result = self._mock_children.get(name)