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

[teor]

  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

[Guinness]

It looks like the only way to replace the platform.linux_distribution method (and advised by python (see ​here) is to use the ​distro package.
If this is okay for you, I can handle this one and attach the patch here (or a link to the commit on my clone of stem).

[atagar]

Thanks Guinness, sounds good! Patch welcome. If something comes up or you'd care for me to give a crack at this I'll probably get to this over the weekend.

[Guinness]

Here you are :) : ​https://github.com/torproject/stem/pull/15

[teor]

All the CI failures are unrelated controller hangs. We're working on them in #29437 and children.

[atagar]

Thanks Guinness! Commented on your PR...

Hi Gu1nness, thank you for your pull request! Our linux_distribution() call is only used for display purposes. I'd prefer if test/task.py simply omits that bit of information when linux_distribution() is unavailable rather than add a new dependency.