added github actions on PR (#178)

* added github actions on PR

* fixed linter issue

Author: sabuhish

.github/workflows/linter.yaml

@@ -3,6 +3,8 @@ name: CI
 on: 
   push:
     branches: '**'
+  pull_request:
+    branches: '**'
 
 jobs:
   ci:

fastapi_mail/schemas.py

@@ -4,7 +4,7 @@
 from typing import Dict, List, Optional, Union
 
 from pydantic import BaseModel, EmailStr, validator
-from starlette.datastructures import UploadFile, Headers
+from starlette.datastructures import Headers, UploadFile
 
 from fastapi_mail.errors import WrongFile