setup: bump decorator minimum version to 4.2.1 to get rid of py3 warning

Avoid verbose warning:

  .../site-packages/decorator.py:95: DeprecationWarning: `formatargspec` is deprecated since Python 3.5. Use `signature` and the `Signature` object directly
    formatvalue=lambda val: "", *argspec)[1:-1]

setup.py

@@ -63,7 +63,7 b' requirements = ['
     "Routes >= 2.0, < 2.5",
     "dulwich >= 0.19.0, < 0.20",
     "mercurial >= 5.1, < 5.3",
-    "decorator >= 3.3.2, < 4.5",
+    "decorator >= 4.2.1, < 4.5",
     "Paste >= 2.0.3, < 3.1",
     "bleach >= 3.0, < 3.2",
     "Click >= 7.0, < 8",