Port to Python 3.

* Use `except Foo as bar` instead of `except Foo, bar` (this is also
  compatible with Python 2).
* Decode command output as UTF-8.

The needed change in debian/control will be just:

```diff
--- a/debian/control
+++ b/debian/control
@@ -9,7 +9,7 @@ Vcs-Browser: https://github.com/porridge/command-runner-applet
 
 Package: command-runner-applet
 Architecture: all
-Depends: ${misc:Depends}, python, python-gi,
+Depends: ${misc:Depends}, python3, python3-gi,
  gir1.2-gtk-3.0, gir1.2-gdkpixbuf-2.0,
  gir1.2-panelapplet-5.0
 Description: panel applet which periodically displays a command output
```