Add explicit dependency on PyYAML.

changelogs/fragments/619-antsibull-fileutils.yml

@@ -1,3 +1,6 @@
 minor_changes:
   - "Add dependency on antsibull-fileutils. Some functionality from antsibull-core is moving there, so we can use it from there directly
      (https://github.com/ansible-community/antsibull/pull/619)."
+bugfixes:
+  - "Add explicit dependency on PyYAML, since we directly use it
+     (https://github.com/ansible-community/antsibull/pull/619)."

pyproject.toml

@@ -37,6 +37,7 @@ dependencies = [
     "aiofiles",
     "aiohttp >= 3.0.0",
     "twiggy",
+    "pyyaml",
     # We rely on deprecated features to maintain compat btw. pydantic v1 and v2
     "pydantic < 3",
     # pydantic already pulls it in, but we use it for TypedDict