Fix NumPy typecheck errors.

Author: edtechre

src/pybroker/eval.py

@@ -453,6 +453,8 @@ def iqr(values: NDArray[np.float64]) -> float:
     x = values[~np.isnan(values)]
     if not len(x):
         return 0
+    q75: float
+    q25: float
     q75, q25 = tuple(np.percentile(x, [75, 25], method="midpoint"))
     return q75 - q25