Rename pygmt.param.Pen to pygmt.params.Pen

Following #3995.

Author: weiji14

pygmt/param/__init__.py

@@ -3,3 +3,4 @@
 """
 
 from pygmt.params.box import Box
+from pygmt.params.pen import Pen

pygmt/params/__init__.py

@@ -1,6 +1,7 @@
 """
 Define the Pen class for specifying pen attributes (width, color, style).
 """
+
 from dataclasses import dataclass
 
 

pygmt/param/pen.py renamed to pygmt/params/pen.py

@@ -1,7 +1,8 @@
 """
 Tests for the Pen param class.
 """
-from pygmt.param import Pen
+
+from pygmt.params import Pen
 
 
 def test_param_pen_width_or_color_or_style_only():