PyYAML bug workaround

Change-Id: I7e4ba7e971f160e64837c067002ab9c98741eeb6
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>
diff --git a/requirements.txt b/requirements.txt
index 25a8e68..a6e6d64 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -78,7 +78,7 @@
     # via -r requirements.in
 python-dateutil==2.8.2
     # via temporalio
-pyyaml==5.4.1
+pyyaml==5.3.1
     # via -r requirements.in
 six==1.16.0
     # via python-dateutil