cluster: move to Python-3.8

[tiborsimko]

This is a meta-issue to migrate all REANA cluster components from Python-3.6 to Python-3.8.

Current status:

$ grep python:3.6 r*/Dockerfile
reana-job-controller/Dockerfile:FROM python:3.6-slim
reana-server/Dockerfile:FROM python:3.6-slim
reana-workflow-controller/Dockerfile:FROM python:3.6-slim
reana-workflow-engine-cwl/Dockerfile:FROM python:3.6-slim
reana-workflow-engine-serial/Dockerfile:FROM python:3.6-slim
reana-workflow-engine-yadage/Dockerfile:FROM python:3.6-slim

The goal:

• All components will start from python:3.8-slim image.

Things to check for:

• The docker base image change could bring or remove some system dependencies, so apt-get commands may need to be updated.

• Python 3.8 may break some of our Python app code, so it may need to be amended.

• Check the old size of e.g. reana-server docker image (e.g. 580MB) and the new size, just to see whether our Docker images are still slim; they should be about the same size.

• .travis.yml to be updated to use Python 3.8.

• setup.py to be updated to declare Python 3.8.

• setup.py may need to be updated as to the lowest versions of various dependency packages.

• requirements.txt needs to be updated afterwards to propagate these changes (via pip-compile).

[alintulu]

I checked the size and they are all in the same size order as before

image	py36	py38
r-w-e-serial	245MB	270MB
r-w-e-cwl	493MB	506MB
r-w-e-yadage	822MB	835MB
r-server	585MB	598MB
r-j-controller	668MB	702MB
r-w-controller	508MB	522MB