upstart-monitor does not work in no-gui mode if Gtk is not available

upstart-monitor attempts to detect if graphical modules are not available and run in no-gui mode (and this can also be explicitly specified on the command line). However, if the modules are not available, Python fails to parse the code because something inherits from Gtk.Window even though Gtk is not available.

# upstart-monitor --no-gui
WARNING: GUI modules not available - falling back to CLI
Traceback (most recent call last):
  File "/usr/bin/upstart-monitor", line 196, in <module>
    class UpstartEventsGui(Gtk.Window):
NameError: name 'Gtk' is not defined

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: upstart-monitor 1.13.2-0ubuntu21.1
ProcVersionSignature: Ubuntu 4.4.0-36.55-generic 4.4.16
Uname: Linux 4.4.0-36-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
BootLog:
 /dev/sda2: clean, 3305789/9920512 files, 35711207/39670016 blocks
 [FAILED] Failed to start Automatically refresh installed snaps.
 See 'systemctl status snapd.refresh.service' for details.
CurrentDesktop: Unity
Date: Fri Sep 2 22:45:21 2016
InstallationDate: Installed on 2014-04-07 (879 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
PackageArchitecture: all
ProcKernelCmdline: BOOT_IMAGE=/boot/vmlinuz-4.4.0-36-generic.efi.signed root=UUID=ad1b3110-ee9a-4dac-839d-604080e0020d ro quiet splash vt.handoff=7
SourcePackage: upstart
UpgradeStatus: Upgraded to xenial on 2016-08-04 (29 days ago)
UpstartBugCategory: System
UpstartRunningSessionVersion: upstart 1.13.2
UpstartRunningSystemJobs:
 Error: command ['initctl', '--system', 'list'] failed with exit code 1: initctl: Name "com.ubuntu.Upstart" does not exist
UpstartRunningSystemVersion: Error: command ['initctl', '--system', 'version'] failed with exit code 1: initctl: Name "com.ubuntu.Upstart" does not exist