fix lint

calculator/expression.py

 """
 Expression module defines the structure of an expression.
 """
-from calculator.operators import Operator
 from typing import Union
+from calculator.operators import Operator
 
 Term: type = int
 Token: type = Union[Operator, Term]

calculator/server.py

-from calculator.calculator import Calculator
+"""
+Run server
+"""
 from fastapi import FastAPI
 from fastapi.requests import Request
 from fastapi.templating import Jinja2Templates
+from calculator.calculator import Calculator
 
 app = FastAPI()
 templates = Jinja2Templates(directory="calculator/templates")