Fix type hint for add_column in lnt.server.db.util

[thopre]

Summary

statement variable in lnt.server.db.util's add_column method is of type
DDLElement. As per [1], the bind argument of its execute() method is
optional and take "an Engine or Connection". This commit updates
add_column's type hint accordingly.

[1]
https://docs.sqlalchemy.org/en/13/core/ddl.html#sqlalchemy.schema.DDLElement.execute

[danilaml]

LGTM

lnt/server/db/util.py

 import sqlalchemy
 import sqlalchemy.ext.compiler
-from sqlalchemy.engine.interfaces import Connectable
+from sqlalchemy.engine import Connection, Engine
 from sqlalchemy.schema import DDLElement
 from sqlalchemy.ext.compiler import compiles
-from typing import Text
+from typing import Text, Union
 
 
 def path_has_no_database_type(path):
     return '://' not in path
 
 
 class _AddColumn(DDLElement):
     def __init__(self, table_name, column):
         self.table_name = table_name
         self.column = column
 
 
 @compiles(_AddColumn)
 def _visit_add_column(element, compiler, **_):
     return ("ALTER TABLE %s ADD COLUMN %s" %
             (compiler.preparer.quote(element.table_name),
              compiler.get_column_specification(element.column)))
 
 
 def add_column(connectable, table_name, column):
-    # type: (Connectable, Text, sqlalchemy.Column) -> None
+    # type: (Union[Engine, Connection, None], Text, sqlalchemy.Column) -> None
     """Add this column to the table named `table_name`.
 
     This is a stopgap to a real migration system.  Inspect the Column pass
     and try to issue an ALTER command to make the column.
 
     :param connectable: to execute on.
     :param table_name: name of table to add the column to.
     :param column: Column to add
     """
     statement = _AddColumn(table_name, column)
     statement.execute(bind=connectable)