buildman: update PyYAML requirement

On Ubuntu 25.04 installing the dependency pyyaml 6.0 fails with

    License classifiers are deprecated

Update PyYAML to the current release.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/tools/buildman/requirements.txt b/tools/buildman/requirements.txt
index 052d0ed..a6b1aec 100644
--- a/tools/buildman/requirements.txt
+++ b/tools/buildman/requirements.txt
@@ -1,5 +1,5 @@
 coverage==6.2
 jsonschema==4.17.3
 pycryptodome==3.20
-pyyaml==6.0
+pyyaml==6.0.2
 yamllint==1.26.3