wx: Use an integral font size.

This is a very-small partial revert of matplotlib#15292, which claimed that wx
supports floats, but this no longer appears to be the case with Python
3.10, and looking at the C++ class constructor [1], should not have ever
been supported.

[1] https://docs.wxwidgets.org/3.0/classwx_font.html

lib/matplotlib/backends/backend_wx.py

@@ -297,10 +297,11 @@ def get_wx_font(self, s, prop):
         font = self.fontd.get(key)
         if font is not None:
             return font
+        size = self.points_to_pixels(prop.get_size_in_points())
         # Font colour is determined by the active wx.Pen
         # TODO: It may be wise to cache font information
         self.fontd[key] = font = wx.Font(  # Cache the font and gc.
-            pointSize=self.points_to_pixels(prop.get_size_in_points()),
+            pointSize=int(size + 0.5),
             family=self.fontnames.get(prop.get_name(), wx.ROMAN),
             style=self.fontangles[prop.get_style()],
             weight=self.fontweights[prop.get_weight()])