add tenacity version 8.0.x to tests (#74)

Author: MikeWooster

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist =
-    requests{216,217,218,219,220,221,222,223,224,225}-tenacity{51,60,61,62,63,70}
+    requests{216,217,218,219,220,221,222,223,224,225}-tenacity{51,60,61,62,63,70,80}
     lint
 
 [testenv]
@@ -21,6 +21,7 @@ deps =
     tenacity62: tenacity>=6.2.0,<6.3.0
     tenacity63: tenacity>=6.3.0,<6.4.0
     tenacity70: tenacity>=7.0.0,<7.1.0
+    tenacity80: tenacity>=8.0.0,<8.1.0
 extras = test
 commands =
     pytest {posargs:{toxinidir}/tests}