Core: Fix up github security issues

This fixes an unused import, an improper use of self and lots and lots
of places where we implicitly return None.  This now explicitly returns
None to improve readability and prevent mixed implicit and explicit
return values.  This should also somewhat aid type checking by humans.

volatility3/framework/plugins/linux/check_syscall.py

@@ -145,7 +145,7 @@ class Check_syscall(plugins.PluginInterface):
             table_info = self._get_table_info(vmlinux, "sys_call_table", ptr_sz)
         except exceptions.SymbolError:
             vollog.error("Unable to find the system call table. Exiting.")
-            return
+            return None
 
         tables = [(table_name, table_info)]