bpo-34500: Fix ResourceWarning in difflib.py (GH-8926)

The change to Tools/scripts/diff.py effectively backports part of
a2637729.
The test code changed in Doc/library/difflib.rst is not present in current 3.x.

Doc/library/difflib.rst

@@ -757,8 +757,10 @@ It is also contained in the Python source distribution, as
        # we're passing these as arguments to the diff function
        fromdate = time.ctime(os.stat(fromfile).st_mtime)
        todate = time.ctime(os.stat(tofile).st_mtime)
-       fromlines = open(fromfile, 'U').readlines()
-       tolines = open(tofile, 'U').readlines()
+       with open(fromfile, 'U') as f:
+           fromlines = f.readlines()
+       with open(tofile, 'U') as f:
+           tolines = f.readlines()
 
        if options.u:
            diff = difflib.unified_diff(fromlines, tolines, fromfile, tofile,

Misc/NEWS.d/next/Tools-Demos/2018-08-25-17-13-24.bpo-34500.Edz41x.rst

+Fix 2 ResourceWarning in difflib.py. Patch by Mickaël Schoentgen.

Tools/scripts/diff.py

@@ -32,8 +32,10 @@ def main():
 
     fromdate = time.ctime(os.stat(fromfile).st_mtime)
     todate = time.ctime(os.stat(tofile).st_mtime)
-    fromlines = open(fromfile, 'U').readlines()
-    tolines = open(tofile, 'U').readlines()
+    with open(fromfile, 'U') as f:
+        fromlines = f.readlines()
+    with open(tofile, 'U') as f:
+        tolines = f.readlines()
 
     if options.u:
         diff = difflib.unified_diff(fromlines, tolines, fromfile, tofile, fromdate, todate, n=n)