requirements.txt: Add flake8
diff --git a/requirements.txt b/requirements.txt
index 05f4b41..fe8a2c1 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,3 +1,5 @@
+flake8
+
 # rst_include tool as GitHub doesn't support `.. include::` when rendering
 # previews.
 rst_include