text: Title: line buffering (buffering=1) warning on Python 3.8
  Body: Since the recent change made in #1014, I got a warning like below when launching images based on Python 3.8 (`python=3.8` in environment.yml).
  
  ```
  /srv/conda/envs/notebook/lib/python3.8/subprocess.py:848: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  ```
  
  I believe this issue is related to [the change](https://bugs.python.org/issue32236) in Python 3.8 where it started complaining whereas previous versions silently ignored `buffering=1` for binary mode. I guess the related code is below:
  
  https://github.com/jupyterhub/repo2docker/blob/a5f5bbbb75a9945d1f8fe8f8ff4844dfd4481742/repo2docker/buildpacks/repo2docker-entrypoint#L40-L46
  
  Not sure if related, I also noticed texts on the console (mostly) lost colors after this recent change; Jupyter log messages used to have colored headings for warning/info/etc, but they are now all monochrome. Yet some texts are still printed in colors (i.e. Julia banner lost colors, but its prompt still has a color).