chore: use non-deprecated license format

Refs: KEH-282

Author: danipran

pyproject.toml

@@ -7,7 +7,8 @@ name = "django_ilmoitin"
 version = "0.7.1"
 description = "Django app for sending notifications."
 readme = "README.md"
-license = {file = "LICENSE"}
+license = "MIT"
+license-files = ["LICENSE"]
 authors = [
     {name = "City of Helsinki", email = "dev@hel.fi"},
 ]