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/treemapsvg.py

@@ -877,10 +877,10 @@ def main(csv_paths, output, *,
 
     # create svg file
     with openio(output, 'w') as f:
-        def writeln(s=''):
-            f.write(s)
-            f.write('\n')
-        f.writeln = writeln
+        def writeln(self, s=''):
+            self.write(s)
+            self.write('\n')
+        f.writeln = writeln.__get__(f)
 
         # yes this is svg
         f.write('<svg '