Update lower bound for dev extra tox requirement

diff --git a/setup.py b/setup.py
index 6a1c6b500bf28091051ec492cd15fa29d471891a..278f27815c894293794161a8a6e1c7832ce36ffa 100644 (file)
--- a/setup.py
+++ b/setup.py
@@ -48,7 +48,7 @@ setup(
         "dev": [
             "coverage>=4.4.2",
             "coveralls>=2.1.0",
-            "tox>=3.14.0",
+            "tox>=3.21.0",
          ],
         "socks": ["requests[socks]>=2.27.0,<3"],
         "test": tests_require,