Update appdirs lower constraint

The appdirs lower constraint of 1.3.0 does not work with isort >=
4.3.5, which references appdirs.system, which wasn't added until
appdirs 1.4.0. Neutron addressed this by increasing the lower
constraint to 1.4.3 [1] so we'll do the same.

[1] 9bfe63cf0786176cd48943d902690198e02fdb70

Change-Id: I5ba07e5504fe786bc7dcb42ef0f43034d5384f6f

lower-constraints.txt

@@ -1,7 +1,7 @@
 alabaster==0.7.10
 alembic==0.8.10
 amqp==2.1.1
-appdirs==1.3.0
+appdirs==1.4.3
 automaton==1.9.0
 Babel==2.3.4
 bashate==0.5.1