Jammy: Restore pyyaml as a requirement

lavacli dependencies were recently changed to be managed by the project
itself. However, as part of that change, PyYAML was also removed which
is not used by lavacli. Instead it uses ruamel.yaml and PyYAML is used
by builder scripts. Rectify this to not break the scripts

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I7b4c02234f3860b516dd74e7b7d0a9746406fb82
diff --git a/jammy-amd64-tf-a-build/requirements_python3.txt b/jammy-amd64-tf-a-build/requirements_python3.txt
index 8253a1e..f7a8f84 100644
--- a/jammy-amd64-tf-a-build/requirements_python3.txt
+++ b/jammy-amd64-tf-a-build/requirements_python3.txt
@@ -1,4 +1,5 @@
 MarkupSafe==1.1.0
+PyYAML==6.0
 Sphinx==2.0.1
 cbor==1.0.0
 configobj==5.0.6
@@ -9,7 +10,6 @@
 m2r==0.2.1
 pyasn1==0.1.9
 pycryptodome==3.6.6
-ruamel.yaml==0.16.12
 sphinx-rtd-theme==0.4.3
 sphinxcontrib-plantuml==0.15
 xmltodict==0.12.0