fix: better error if requires is not a set

Signed-off-by: Henry Schreiner <[email protected]>

src/repo_review/processor.py

@@ -199,6 +199,11 @@ def process(
     graph: dict[str, set[str]] = {
         n: getattr(t, "requires", set()) for n, t in tasks.items()
     }
+    for name, s in graph.items():
+        if not isinstance(s, set):
+            msg = f"requires must be a set, got {s!r} for {name!r}"  # type: ignore[unreachable]
+            raise TypeError(msg)
+
     # Keep track of which checks have been completed
     completed: dict[str, str | None] = {}