fix python interpreter

alacritty_config.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 import os
 import sys
 

alacritty_config_gui.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 import os
 import platform
 import sys
@@ -10,7 +10,7 @@ try:
 except:
     print('alacritty-config-gui depends on PyQt5. I couldn’t locate PyQt5.')
     print('Sorry about that.')
-    print('\nRunning `pip3 install PyQt5` could help.')
+    print('\nRunning `pip install PyQt5` could help.')
     sys.exit(1)
 
 
@@ -502,7 +502,7 @@ class General(ConfigWidget):
     def __init__(self, config):
         super().__init__()
         self.widgets['background_opacity'] = QtWidgets.QDoubleSpinBox()
-        self.widgets['background_opacity'].setValue(config.get('background_opacity'))
+        self.widgets['background_opacity'].setValue(config.get('background_opacity') or 0)
         self.widgets['cursor_style'] = QtWidgets.QComboBox()
         for option in self.cursor_style_options:
             self.widgets['cursor_style'].addItem(option)