setup: support Mercurial 6.1 for continuously improved Python 3 support

setup.py

@@ -63,7 +63,7 b' requirements = ['
     "URLObject >= 2.3.4, < 2.5",
     "Routes >= 2.0, < 2.5",
     "dulwich >= 0.19.0, < 0.20",
-    "mercurial >= 5.2, < 5.10",
+    "mercurial >= 5.2, < 6.2",
     "decorator >= 4.2.1, < 4.5",
     "Paste >= 2.0.3, < 3.5",
     "bleach >= 3.2, < 4.2",