Fix invalid string escape sequences

Python 3.12+ emits SyntaxWarning for unrecognised backslash escapes (and
these will eventually become SyntaxErrors). Convert the affected string
literals to raw strings so the escapes are explicit:

  - scriptinterface.py: filename filter regexes (\.der, \.b64, ...).
  - obok.py: MAC-address detection regexes (\s, \-) and the title
    sanitiser ([^\s\w]).
  - flatxml2svg.py: the emitted JavaScript snippet contains \? and \d but
    also a trailing newline, so the regex part is made a raw string and
    the "\n" is concatenated separately.

All changes are value-preserving (an unrecognised escape already evaluated
to the backslash plus the character); verified the emitted JS string and
the compiled regexes are unchanged, and that both plugins now compile with
zero SyntaxWarnings.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

DeDRM_plugin/flatxml2svg.py

@@ -199,7 +199,7 @@ def convert2SVG(gdict, flat_xml, pageid, previd, nextid, svgDir, raw, meta_array
         mlst.append('<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" ><head>\n')
         mlst.append('<title>Page %d - %s by %s</title>\n' % (pageid, meta_array['Title'],meta_array['Authors']))
         mlst.append('<script><![CDATA[\n')
-        mlst.append('function gd(){var p=window.location.href.replace(/^.*\?dpi=(\d+).*$/i,"$1");return p;}\n')
+        mlst.append(r'function gd(){var p=window.location.href.replace(/^.*\?dpi=(\d+).*$/i,"$1");return p;}' + '\n')
         mlst.append('var dpi=%d;\n' % scaledpi)
         if (previd) :
             mlst.append('var prevpage="page%04d.xhtml";\n' % (previd))