DeprecationWarning from docstrings

[josch]

With Python 3.7 I'm getting warnings like:

/usr/lib/python3/dist-packages/pdfrw/objects/pdfstring.py:265: DeprecationWarning: invalid escape sequence \(

The following patch fixes the problem:

--- /usr/lib/python3/dist-packages/pdfrw/objects/pdfstring.py.bak	2019-07-01 10:45:42.857128581 +0200
+++ /usr/lib/python3/dist-packages/pdfrw/objects/pdfstring.py	2019-07-01 10:46:35.325658317 +0200
@@ -117,7 +117,7 @@
 in literal strings is to not escape parentheses.  This only works, and is
 only allowed, when the parentheses are properly balanced.  For example,
 "((Hello))" is a valid encoding for a literal string, but "((Hello)" is not;
-the latter case should be encoded "(\(Hello)"
+the latter case should be encoded "(\\(Hello)"
 
 Encoding text into strings
 ==========================
@@ -372,12 +372,12 @@
             Possible string escapes from the spec:
             (PDF 1.7 Reference, section 3.2.3, page 53)
 
-                1. \[nrtbf\()]: simple escapes
+                1. \\[nrtbf\\()]: simple escapes
                 2. \\d{1,3}: octal. Must be zero-padded to 3 digits
                     if followed by digit
-                3. \<end of line>: line continuation. We don't know the EOL
+                3. \\<end of line>: line continuation. We don't know the EOL
                     marker used in the PDF, so accept \r, \n, and \r\n.
-                4. Any other character following \ escape -- the backslash
+                4. Any other character following \\ escape -- the backslash
                     is swallowed.
         """
         result = (self.unescape_func or self.init_unescapes())(self[1:-1])