Tensorboard results in error about html5lib when used with python 3.6m

[CMCDragonkai]

Issue description

Installing tensorflow-tensorboard gives this error:

error: html5lib-0.9999999 not supported for interpreter python3.6m

Steps to reproduce

Using this shell.nix:

{
  pkgs ? import (fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/6c064e6b1f34a8416f990db0cc617a7195f71588.tar.gz) {}
}:
  with pkgs;
  let
    python = python36;
  in
    python.pkgs.buildPythonApplication {
      name = "testenv";
      buildInputs =  (with python.pkgs; [
        tensorflowWithCuda
        tensorflow-tensorboard
      ]);
    }

The hash is the current latest hash on nixpkgs-unstable.

@abbradar

[dotlambda]

Tensorboard officially only supports python up to version 3.5, see e.g. tensorflow/tensorboard#427 (comment) (maybe support has been added in the meantime?).
However, once a new version of tensorboard inlcuding tensorflow/tensorboard#1142 is released, this may be fixed. It will also allow us to drop the old versions of html5lib and bleach from pythonPackages.

[abbradar]

It is as said -- Tensorboard is not supported on Python 3.6. We hack around it allowing Tensorflow to be installed without Tensorboard in this case.