Restrict Pydantic dependency to version 1.x or 2.x

The `json()` method was renamed to `model_dump_json()` in Pydantic 2.0.
The old name is still supported in Pydantic 2.x, but is scheduled for
removal in Pydantic 3.0.

https://docs.pydantic.dev/2.5/migration/#changes-to-pydanticbasemodel

Closes #113

Author: mthuurne

poetry.lock

@@ -37,7 +37,7 @@ classifiers = [
 [tool.poetry.dependencies]
 python = "^3.8"
 Django = ">=2,<6"
-pydantic = "*"
+pydantic = ">=1,<3"
 typing-extensions = "^4.5"
 
 [tool.poetry.group.build.dependencies]