Use open() instead of the file constructor

This is required to make this script work with Python 3, but even
in Python 2 open is preferred over file():
https://docs.python.org/2/library/functions.html#file

Bug: 941669
Change-Id: I4738fc9bad81464c9ca5c9c15a3c51e9ee3f945d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1745488
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685372}

third_party/binutils/download.py

@@ -31,13 +31,13 @@ import detect_host_arch
 
 
 def ReadFile(filename):
-  with file(filename, 'r') as f:
+  with open(filename, 'r') as f:
     return f.read().strip()
 
 
 def WriteFile(filename, content):
   assert not os.path.exists(filename)
-  with file(filename, 'w') as f:
+  with open(filename, 'w') as f:
     f.write(content)
     f.write('\n')