Meta: Avoid an IndexError with .get() instead of indexing in the weeknotes script

Replace indexing into the os.environ dictionary with a .get() call (and a default)
to avoid an IndexError

scripts/weeknotes

@@ -39,7 +39,7 @@ def parse_args(argv, *a, **kw):
     )
     edit_command.add_argument(
         '--editor', '-e',
-        default=os.environ['EDITOR'] or 'nvim',
+        default=os.environ.get('EDITOR', 'nvim'),
     )
     edit_command.add_argument('--week', '-w')
     edit_command.set_defaults(handler=handle_edit_command)