setup: add versioned dependency on pyaml

In commit 24131333ac ("trivial: disable
YAML loading warning"), an assumption was made that the version of pyaml
present was 5.1 or later; see https://msg.pyyaml.org/load .

Version the dependency.

tools/python-deps/setup.py

@@ -36,7 +36,7 @@ DEPS = [
     'pyelftools',
     'sh',
     'pexpect',
-    'pyaml',
+    'pyaml>=5.1',
     'jsonschema',
     'pyfdt',
     'cmake-format==0.4.5',