Officially require Python 3.8

Our code is still compatible with Python 3.5 at the time of writing, but we
don't want to commit to that.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/python3.8.txt b/ChangeLog.d/python3.8.txt
new file mode 100644
index 0000000..32a7c09
--- /dev/null
+++ b/ChangeLog.d/python3.8.txt
@@ -0,0 +1,2 @@
+Requirement changes
+   * Officially require Python 3.8 now that earlier versions are out of support.