Ensure '\' are still escaped when creating patch file

I got an error using conda skeleton. It was caused by '\' not being escaped correctly when creating the patch file for distutils.core.py

I think this pr solves this. 

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patching file `core.py'
Hunk #1 succeeded at 169 with fuzz 2 (offset 2 lines).
Traceback (most recent call last):
  File "setup.py", line 39, in <module>
    url="http://github.com/Julian/jsonschema",
  File "C:\Users\Morten\Anaconda\envs\_build\lib\distutils\core.py", line 192, in setup
    with io.open(os.path.join("c:\users\morten\appdata\local\temp\tmpnt7iciconda_skeleton_jsonschema", "pkginfo.yaml"), 'w', encodin
g='utf-8') as fn:
IOError: [Errno 22] Invalid argument: 'c:\\users\\morten\x07ppdata\\local\temp\tmpnt7iciconda_skeleton_jsonschema\\pkginfo.yaml'
$PYTHONPATH = c:\users\morten\appdata\local\temp\tmpnt7iciconda_skeleton_jsonschema\jsonschema-2.3.0
Error: command failed: C:\Users\Morten\Anaconda\envs\_build\python.exe setup.py install