eth/precompiles/modexp: fix complexity calc (length^2, not 2^2).

There was a typo in the "complexity" calculation function, a
special interim value from EIP-198 used to determine total gas
use.

The code path was never previously exercised.

NOTE: previous-commit "fixtures bump" was to commit:

9b1f07c58a70d1b17c4489c49eb9bebf4a27d290

eth/precompiles/modexp.py

@@ -35,7 +35,7 @@ def _compute_complexity(length):
             length ** 2 // 4 + 96 * length - 3072
         )
     else:
-        return 2 ** 2 // 16 + 480 * length - 199680
+        return length ** 2 // 16 + 480 * length - 199680
 
 
 def _extract_lengths(data):

tests/json-fixtures/test_state.py

@@ -138,11 +138,6 @@ def expand_fixtures_forks(all_fixtures):
 # Ideally, this list should be empty.
 # WHEN ADDING ENTRIES, ALWAYS PROVIDE AN EXPLANATION!
 INCORRECT_UPSTREAM_TESTS = {
-    # Upstream seems to specify that the precompile call fails, but `py-evm`
-    # handles it just fine.
-    # * https://github.com/ethereum/py-evm/pull/1224#issuecomment-417351843
-    # * https://github.com/ethereum/tests/pull/405#issuecomment-417855812
-    ('stReturnDataTest/modexp_modsize0_returndatasize.json', 'modexp_modsize0_returndatasize', 'Byzantium', 4),  # noqa: E501
 }