Add combination_pairs helper function

Wrapper function for itertools.combinations_with_replacement, with
explicit cast due to imprecise typing with older versions of mypy.

Signed-off-by: Werner Lewis <werner.lewis@arm.com>

tests/scripts/generate_bignum_tests.py

@@ -73,6 +73,17 @@ def hex_to_int(val: str) -> int:
 def quote_str(val) -> str:
     return "\"{}\"".format(val)
 
+def combination_pairs(values: List[T]) -> List[Tuple[T, T]]:
+    """Return all pair combinations from input values.
+
+    The return value is cast, as older versions of mypy are unable to derive
+    the specific type returned by itertools.combinations_with_replacement.
+    """
+    return typing.cast(
+        List[Tuple[T, T]],
+        list(itertools.combinations_with_replacement(values, 2))
+    )
+
 
 class BignumTarget(test_generation.BaseTarget, metaclass=ABCMeta):
     #pylint: disable=abstract-method
@@ -165,10 +176,7 @@ class BignumOperation(BignumTarget, metaclass=ABCMeta):
         Combinations are first generated from all input values, and then
         specific cases provided.
         """
-        yield from typing.cast(
-            Iterator[Tuple[str, str]],
-            itertools.combinations_with_replacement(cls.input_values, 2)
-        )
+        yield from combination_pairs(cls.input_values)
         yield from cls.input_cases
 
     @classmethod
@@ -215,14 +223,11 @@ class BignumAdd(BignumOperation):
     symbol = "+"
     test_function = "mbedtls_mpi_add_mpi"
     test_name = "MPI add"
-    input_cases = typing.cast(
-        List[Tuple[str, str]],
-        list(itertools.combinations_with_replacement(
+    input_cases = combination_pairs(
         [
             "1c67967269c6", "9cde3",
             "-1c67967269c6", "-9cde3",
-            ], 2
-        ))
+        ]
     )
 
     def result(self) -> str: