Sanitize default filename.

I would actually think that we should not do any sanitization ("you get
what you asked for"), but given that there was some opposition last time
I tried to restore the space in the default filename ("Figure 1.png"),
even though the space is totally innocuous for the filesystem, we may as
well fix the filenames that are *actually* problematic.

Co-authored-by: Greg Lucas <greg.m.lucas@gmail.com>

MNT: remove unneeded access of savefig.directory

Author: anntzer

lib/matplotlib/backend_bases.py

@@ -2241,15 +2241,22 @@ def get_default_filetype(cls):
 
     def get_default_filename(self):
         """
-        Return a string, which includes extension, suitable for use as
-        a default filename.
-        """
-        basename = (self.manager.get_window_title() if self.manager is not None
-                    else '')
-        basename = (basename or 'image').replace(' ', '_')
-        filetype = self.get_default_filetype()
-        filename = basename + '.' + filetype
-        return filename
+        Return a suitable default filename, including the extension.
+        """
+        default_basename = (
+            self.manager.get_window_title()
+            if self.manager is not None
+            else ''
+        )
+        default_basename = default_basename or 'image'
+        # Characters to be avoided in a NT path:
+        # https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#naming_conventions
+        removed_chars = \
+            {"posix": " /\0", "nt": r'<>:"/\|?*\0'}.get(os.name, "_")
+        default_basename = default_basename.translate(
+            {ord(c): "_" for c in removed_chars})
+        default_filetype = self.get_default_filetype()
+        return f'{default_basename}.{default_filetype}'
 
     @_api.deprecated("3.8")
     def switch_backends(self, FigureCanvasClass):