Fix text position

With the anchor ms (middle baseline) the text is not exactly at the same
position than before. I changed it to md (middle descender) to have the
same position.
See
https://pillow.readthedocs.io/en/stable/handbook/text-anchors.html#text-anchors

barcode/writer.py

@@ -443,7 +443,7 @@ def _paint_text(self, xpos, ypos):
                     mm2px(ypos, self.dpi),
                 )
                 self._draw.text(
-                    pos, subtext, font=font, fill=self.foreground, anchor="ms"
+                    pos, subtext, font=font, fill=self.foreground, anchor="md"
                 )
                 ypos += pt2mm(self.font_size) / 2 + self.text_line_distance