Fix up a couple extra typing issues, improving code quality.

volatility/framework/automagic/pdbscan.py

@@ -255,9 +255,9 @@ class KernelPDBScanner(interfaces.automagic.AutomagicInterface):
         for virtual_layer_name in potential_kernels:
             kernels = potential_kernels[virtual_layer_name]
             virtual_config_path = context.memory[virtual_layer_name].config_path
-            if virtual_layer_name and isinstance(context.memory[virtual_layer_name], layers.intel.Intel):
+            vlayer = context.memory[virtual_layer_name]
+            if virtual_layer_name and isinstance(vlayer, layers.intel.Intel):
                 # TODO: Verify this is a windows image
-                vlayer = context.memory[virtual_layer_name]
                 join = interfaces.configuration.path_join
                 physical_layer_name = context.config.get(join(vlayer.config_path, 'memory_layer'), None)
                 kvo_path = join(virtual_config_path, 'kernel_virtual_offset')
@@ -271,7 +271,7 @@ class KernelPDBScanner(interfaces.automagic.AutomagicInterface):
                     else:
                         kvo = kernel['mz_offset'] + (1 << (vlayer.bits_per_register - 1))
                     try:
-                        kvp = context.memory[virtual_layer_name].mapping(kvo, 0)
+                        kvp = vlayer.mapping(kvo, 0)
                         if (any([(p == kernel['mz_offset'] and l == physical_layer_name) for (_, p, _, l) in
                                  kvp])):
                             valid_kernels[virtual_layer_name] = (kvo, kernel)

volatility/framework/interfaces/symbols.py

@@ -99,6 +99,10 @@ class SymbolSpaceInterface(collections.abc.Mapping):
     def has_enumeration(self, name: str) -> bool:
         """Determines whether an enumeration choice exists in the contained symbol tables"""
 
+    @abstractmethod
+    def append(self, value: interfaces.symbols.BaseSymbolTableInterface) -> None:
+        """Adds a symbol_list to the end of the space"""
+
 
 class BaseSymbolTableInterface(validity.ValidityRoutines):
     """The base interface, inherited by both NativeTables and SymbolTables