Add tox tests for osm charms in devops

Change-Id: Ibbf1154e514bf8a91ed6d19b6bbf5fce0079da0c
Signed-off-by: David Garcia <david.garcia@canonical.com>
diff --git a/installers/charm/prometheus/tox.ini b/installers/charm/prometheus/tox.ini
index a7625a6..5491c07 100644
--- a/installers/charm/prometheus/tox.ini
+++ b/installers/charm/prometheus/tox.ini
@@ -26,7 +26,7 @@
 skip_missing_interpreters = False
 
 [testenv]
-basepython = python3
+basepython = python3.8
 setenv =
   PYTHONHASHSEED=0
   PYTHONPATH = {toxinidir}/src