setup: bump dulwich minimum version to 0.19.0 to get good py3 support

Fix failure in test_compare_forks_on_branch_extra_commits_git ... but make a
major bump to make sure we get good py3 support in this cruicial and complex
low level library.

setup.py

@@ -61,7 +61,7 b' requirements = ['
     "docutils >= 0.11, < 0.15",
     "URLObject >= 2.3.4, < 2.5",
     "Routes >= 2.0, < 2.5",
-    "dulwich >= 0.14.1, < 0.20",
+    "dulwich >= 0.19.0, < 0.20",
     "mercurial >= 5.1, < 5.3",
     "decorator >= 3.3.2, < 4.5",
     "Paste >= 2.0.3, < 3.1",