setup.py: limit elasticsearch max version

* The latest 7.15.0 release has started to deprecate things for the
  upcoming 8.0.0 release, and mypy started complaining about some return
  types.
* Instead of fixing the signatures to be compatible with both versions
  put a max version limit for now, we'll deal with the upgrade when the
  time will come, Debian most recent version is 7.1.0.

Change-Id: I185a34585e4eac400f84fadffc5f364f27b2313c

setup.py

@@ -10,7 +10,7 @@
     "conftool>=1.0.1",
     "cumin>=3.0.2",
     "dnspython>=1.16.0",
-    "elasticsearch>=5.0.0,<8.0.0",
+    "elasticsearch>=5.0.0,<7.15.0",
     "elasticsearch-curator>=5.0.0",
     "pymysql>=0.9.3",
     "pynetbox>=5.0.7",