BUG : make sure lw is a float

Explicitly cast linewidth to a float in `set_linewidth` methods
of `Line2D` and `Patch`

Closes matplotlib#4306

Author: tacaswell

lib/matplotlib/lines.py

@@ -919,7 +919,7 @@ def set_linewidth(self, w):
 
         ACCEPTS: float value in points
         """
-        self._linewidth = w
+        self._linewidth = float(w)
 
     def set_linestyle(self, linestyle):
         """

lib/matplotlib/patches.py

@@ -334,7 +334,7 @@ def set_linewidth(self, w):
         """
         if w is None:
             w = mpl.rcParams['patch.linewidth']
-        self._linewidth = w
+        self._linewidth = float(w)
 
     def set_lw(self, lw):
         """alias for set_linewidth"""