Update pyjwt to 2.8.0 version

[szmktk]

This pull request updates pyjwt to its most recent version as of today.

I also decided to ditch djangorestframework-jwt-4 package which seems to be abandoned and was not compatible with pyjwt 2.8.0 ("module 'jwt' has no attribute 'ExpiredSignature'" error)

This update is important since pyjwt 1.7.1 was released in 2018 and contains CWE-327. As of now this is the only high severity Dependabot alert that we have on backend side of Zeton.

[szmktk]

@MateuszBelczowski I removed some code which I thought is fossil 🦴 from users/serializers.py and entire zeton_backend/utils.py. Could you verify if this is safe? This code was added in 2020 and I'm not sure if it's used at all 🤔

Also the response schema of /api/token-auth changed:

before:
❯ curl -s -X "POST" "http://localhost:8000/api/token-auth/" -H "accept: application/json" -H "Content-Type: application/json" -d '{"username": "opiekun1","password": "opiekun1"}' | jq
{
  "token": "eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VyX2lkIjozLCJ1c2VybmFtZSI6Im9waWVrdW4xIiwiZXhwIjoxNzE5MDY4ODYyLCJlbWFpbCI6Im1hcmVrLXpldG9uQG1haWxpbmF0b3IuY29tIn0.GqA3koxMnoMDMuDzQoa_XtDMnce9paCD1bNUdgPt2Cg",
  "user": {
    "id": 3,
    "username": "opiekun1"
  }
}

after:
❯ curl -s -X "POST" "http://localhost:8000/api/token-auth/" -H "accept: application/json" -H "Content-Type: application/json" -d '{"username": "opiekun1","password": "opiekun1"}' | jq
{
  "refresh": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ0b2tlbl90eXBlIjoicmVmcmVzaCIsImV4cCI6MTcxOTE1MDcwNSwiaWF0IjoxNzE5MDY0MzA1LCJqdGkiOiIyMTAyMDU1NjFkNTM0ZTFhOTZlYTk5ODYwZGI5MmZkZSIsInVzZXJfaWQiOjN9.sbEF5Ziw6_LLRU2Hqn94xeykAn9Q1506t5fOzejDdGU",
  "access": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ0b2tlbl90eXBlIjoiYWNjZXNzIiwiZXhwIjoxNzE5MDcxNTA1LCJpYXQiOjE3MTkwNjQzMDUsImp0aSI6IjNlMmYzMTIwMzBlNjQ1ZDY5NjI1YzdmMWQ4OTI3YWNlIiwidXNlcl9pZCI6M30.Wg49AmDeFeN5vRdFGOvv8qWFxjdwA6T8_UBli7CdVpM"
}

Is that acceptable? I verified that the token works - I am able to get 200 OK response from GET /api/students/ endpoints.