tests: Add tests for the function dpss from singal.windows.

This function has a few different overloads so it is nice to have tests
for them.

tests/signal/windows/test_windows.pyi

@@ -0,0 +1,14 @@
+from typing import TypeAlias
+from typing_extensions import assert_type
+
+import numpy as np
+from scipy.signal.windows import dpss
+
+_Array_f8_1d: TypeAlias = np.ndarray[tuple[int], np.dtype[np.float64]]
+_Array_f8_2d: TypeAlias = np.ndarray[tuple[int, int], np.dtype[np.float64]]
+
+# test dpss function overloads
+assert_type(dpss(64, 3), _Array_f8_1d)
+assert_type(dpss(64, 3, 2), _Array_f8_2d)
+assert_type(dpss(64, 3, return_ratios=True), tuple[_Array_f8_1d, np.float64])
+assert_type(dpss(64, 3, 2, return_ratios=True), tuple[_Array_f8_2d, _Array_f8_1d])