Python 3.6 invalid escape sequence deprecation fixes (#177)

https://docs.python.org/3/whatsnew/3.6.html#deprecated-python-behavior
diff --git a/tests/test_c_parser.py b/tests/test_c_parser.py
index 95ae865..0835013 100755
--- a/tests/test_c_parser.py
+++ b/tests/test_c_parser.py
@@ -1718,7 +1718,7 @@
 
         self.assertTrue(isinstance(p.ext[0], Typedef))
         self.assertEqual(p.ext[0].coord.line, 213)
-        self.assertEqual(p.ext[0].coord.file, "D:\eli\cpp_stuff\libc_include/stddef.h")
+        self.assertEqual(p.ext[0].coord.file, r"D:\eli\cpp_stuff\libc_include/stddef.h")
 
         self.assertTrue(isinstance(p.ext[-1], FuncDef))
         self.assertEqual(p.ext[-1].coord.line, 15)