Add support for python 3.12

- Replace `assertEquals` (deprecated) with `assertEqual`.
- Replace `self.assert_` with `self.assertTrue`.
- Fix docstrings in `cpplint.py` that contain invalid escape
  sequences to be raw strings.

cpplint/cpplint.py

@@ -58,7 +58,7 @@
 from six.moves import xrange
 
 
-_USAGE = """
+_USAGE = r"""
 Syntax: cpplint.py [--verbose=#] [--output=vs7] [--filter=-x,+y,...]
                    [--counting=total|toplevel|detailed] [--root=subdir]
                    [--linelength=digits] [--headers=x,y,...]
@@ -1112,7 +1112,7 @@ def FullName(self):
     return os.path.abspath(self._filename).replace('\\', '/')
 
   def RepositoryName(self):
-    """FullName after removing the local path to the repository.
+    r"""FullName after removing the local path to the repository.
 
     If we have a real absolute path name here we can try to do something smart:
     detecting the root of the checkout and truncating /path/to/checkout from