access_token: use os.urandom to generate token

Using same method as python 3.6's secrets module.
3 jobs for for_testrunner_1
Status Job ID Name Coverage
  External
running ERP5.CodingStyleTest-TestRunner1

37494:05:29

running ERP5.PerformanceTest-TestRunner1

37494:04:27

running ERP5.UnitTest-TestRunner1

37494:05:24