scripts: Adopt __get__ binding for write/writeln methods

This actually binds our custom write/writeln functions as methods to the
file object:

  def writeln(self, s=''):
      self.write(s)
      self.write('\n')
  f.writeln = writeln.__get__(f)

This doesn't really gain us anything, but is a bit more correct and may
be safer if other code messes with the file's internals.

scripts/codemap.py

@@ -951,11 +951,11 @@ def main_(ring, paths, *,
         label=False,
         no_label=False,
         **args):
-    # give ring an writeln function
-    def writeln(s=''):
-        ring.write(s)
-        ring.write('\n')
-    ring.writeln = writeln
+    # give ring a writeln function
+    def writeln(self, s=''):
+        self.write(s)
+        self.write('\n')
+    ring.writeln = writeln.__get__(ring)
 
     # figure out what color should be
     if color == 'auto':