Remove legacy "from __future__ import with_statement" lines.

Tools/gdb/libpython.py

@@ -44,7 +44,7 @@ The module also extends gdb with some python-specific commands.
 # NOTE: some gdbs are linked with Python 3, so this file should be dual-syntax
 # compatible (2.6+ and 3.0+).  See #19308.
 
-from __future__ import print_function, with_statement
+from __future__ import print_function
 import gdb
 import os
 import locale

Tools/hg/hgtouch.py

@@ -7,7 +7,6 @@ syntax of make rules.
 
 In addition to the dependency syntax, #-comments are supported.
 """
-from __future__ import with_statement
 import errno
 import os
 import time

Tools/pybench/With.py

@@ -1,4 +1,3 @@
-from __future__ import with_statement
 from pybench import Test
 
 class WithFinally(Test):

Tools/scripts/generate_opcode_h.py

@@ -1,7 +1,5 @@
 # This script generates the opcode.h header file.
 
-from __future__ import with_statement
-
 import sys
 header = """/* Auto-generated by Tools/scripts/generate_opcode_h.py */
 #ifndef Py_OPCODE_H