Run flake8 during releases.

releaser.conf

@@ -2,7 +2,7 @@
 news_file: "NEWS"
 timeout_days: 5
 tag_name: "dulwich-$VERSION"
-verify_command: "make check"
+verify_command: "flake8 && make check"
 update_version {
   path: "setup.py"
   match: "^dulwich_version_string = '(.*)'$"