Merge ~peter-sabaini/charm-nagios:fix/lp1893434 into ~nagios-charmers/charm-nagios:master

[Peter Sabaini]

[Facundo Ciccioli (fandanbango)]

I've verified that without this MR, running upgrade-charm will leave the charm in error state.

Including the MR renders the charm properly upgradable.

review: Approve

[Paul Goins (vultaire)]

review: Approve

[Paul Goins (vultaire)]

Released as cs:~llama-charmers-next/nagios-8

diff --git a/hooks/upgrade_charm.py b/hooks/upgrade_charm.py
index ca410ef..4152e53 100755
--- a/hooks/upgrade_charm.py
+++ b/hooks/upgrade_charm.py
@@ -13,6 +13,13 @@ import stat
 import string
 import subprocess
 
+try:
+    from enum import Enum  # noqa: F401
+except ImportError:
+    subprocess.check_call(
+        "DEBIAN_FRONTEND=noninteractive apt-get -qy install python-enum34", shell=True
+    )
+
 from charmhelpers import fetch
 from charmhelpers.contrib import ssl
 from charmhelpers.core import hookenv, host