BaseTools:Remove deprecated ast.Str import for Python 3.14 compatibility

- Str is not being used in FvHeader.py
- Removed import of ast.Str as it was removed in Python 3.14.
- Ensures compatibility with Python 3.14 and later.
- https://docs.python.org/3/whatsnew/3.14.html#id9

This addresses ImportError caused by removal of deprecated AST classes
including ast.Str.

Signed-off-by: Ashraf Ali S <ashraf.ali.s@intel.com>

BaseTools/Source/Python/FirmwareStorageFormat/FvHeader.py

@@ -4,7 +4,6 @@
 # Copyright (c) 2021-, Intel Corporation. All rights reserved.<BR>
 # SPDX-License-Identifier: BSD-2-Clause-Patent
 ##
-from ast import Str
 from struct import *
 from ctypes import *
 from FirmwareStorageFormat.Common import *