Avoid RunTime warning on startup with docker

[francbartoli]

Description
A warning is emitted by gunicorn at bootstrap:

[2020-04-30 15:51:57 +0000] [12] [INFO] Starting gunicorn 19.9.0
[2020-04-30 15:51:57 +0000] [12] [INFO] Listening at: http://0.0.0.0:80 (12)
[2020-04-30 15:51:57 +0000] [12] [INFO] Using worker: gevent
/usr/lib/python3.8/os.py:1023: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  return io.open(fd, *args, **kwargs)

Steps to Reproduce
Steps to reproduce the behavior:

Start pygeoapi with docker.

Expected behavior

This could be resolved by upgrading gunicorn as per this issue

Screenshots/Tracebacks
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Environment

• OS: Docker
• Python version: 3.8
• pygeoapi version: master

Additional context
Add any other context about the problem here.