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 `mypy`:

    eth/vm/opcode.py:40: error: Return type "Opcode" of "as_opcode" incompatible with return type "Type[Opcode]" in supertype "OpcodeAPI"

This commit has been cherry-picked from previous PR:

07b4398

Author: veox

eth/abc.py

@@ -886,9 +886,9 @@ 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.
+        Class factory method for turning vanilla functions into Opcodes.
         """
         ...