ABC: return same from OpcodeAPI's as_opcode() as from Opcode.

99.99% cargo-culting.

I don't understand the purpose of this structuring, but the function
signatures don't match, and this is the only permutation I could
find between OpcodeAPI, Opcode and FRONTIER_OPCODES that stops
producing an error when running `tox`.

Haven't run actual tests yet.

eth/abc.py

@@ -775,7 +775,7 @@ def __call__(self, computation: 'ComputationAPI') -> None:
     def as_opcode(cls: Type[T],
                   logic_fn: Callable[['ComputationAPI'], None],
                   mnemonic: str,
-                  gas_cost: int) -> Type[T]:
+                  gas_cost: int) -> T:
         """
         Class factory method for turning vanilla functions into Opcode classes.
         """