rpm: add R: python3-yaml, as needed by qvm-backup tool

Fixes QubesOS/qubes-issues#4666

rpm_spec/qubes-core-admin-client.spec.in

@@ -14,6 +14,7 @@ BuildRequires:	python%{python3_pkgversion}-devel
 BuildRequires:	python%{python3_pkgversion}-sphinx
 BuildRequires:	python%{python3_pkgversion}-dbus
 Requires:   python%{python3_pkgversion}-qubesadmin
+Requires:   python%{python3_pkgversion}-yaml
 BuildArch:  noarch
 Source0: %{name}-%{version}.tar.gz