ABC: add missing chain_context to VirtualMachineAPI's constructor.

This silences `mypy` error:

eth/chains/base.py:250: error: Unexpected keyword argument "chain_context" for "VirtualMachineAPI"
eth/abc.py:2270: note: "VirtualMachineAPI" defined here

eth/abc.py

@@ -2267,7 +2267,10 @@ class VirtualMachineAPI(ConfigurableAPI):
     extra_data_max_bytes: ClassVar[int]
 
     @abstractmethod
-    def __init__(self, header: BlockHeaderAPI, chaindb: ChainDatabaseAPI) -> None:
+    def __init__(self,
+                 header: BlockHeaderAPI,
+                 chaindb: ChainDatabaseAPI,
+                 chain_context: ChainContextAPI) -> None:
         """
         Initialize the virtual machine.
         """