Fix TypeError in reconfigure() of terminal.py (line 753)

Resolved a bug where `self.bgcolor.alpha` expected a number but
received a string, causing a TypeError. The issue was traced to
background darkness configuration handling.

terminatorlib/terminal.py

@@ -750,7 +750,7 @@ def reconfigure(self, _widget=None):
             self.bgcolor.parse(self.config['background_color'])
 
         if self.config['background_type'] in ('transparent', 'image'):
-            self.bgcolor.alpha = self.config['background_darkness']
+            self.bgcolor.alpha = float(self.config['background_darkness'])
         else:
             self.bgcolor.alpha = 1