Fix invalid escape sequences

Fixes SyntaxWarnings:

.../site-packages/pdfrw/objects/pdfstring.py:6: SyntaxWarning: invalid escape sequence '\('
.../site-packages/pdfrw/objects/pdfstring.py:367: SyntaxWarning: invalid escape sequence '\['

Author: The-Compiler

pdfrw/objects/pdfstring.py

@@ -3,7 +3,7 @@
 #                    2016 James Laird-Wah, Sydney, Australia
 # MIT license -- See LICENSE.txt for details
 
-"""
+r"""
 
 ================================
 PdfString encoding and decoding
@@ -364,7 +364,7 @@ def init_unescapes(cls):
         return unescape_func
 
     def decode_literal(self):
-        """ Decode a PDF literal string, which is enclosed in parentheses ()
+        r""" Decode a PDF literal string, which is enclosed in parentheses ()
 
             Many pdfrw users never decode strings, so defer creating
             data structures to do so until the first string is decoded.