stem tests expect platform.linux_distribution, which does not exist in python 3.8

  python version...                          3.8.0a3+
  operating system...                        failed
module 'platform' has no attribute 'linux_distribution'
The command "./run_tests.py --unit" exited with 1.

https://travis-ci.org/teor2345/stem/jobs/536613179#L1529