quickfix: dependabot merged incompatible eventlet

quickfix, eventlet back to 0.30.2 for now - see also: benoitc/gunicorn#2581 (not yet in version)

Error with eventlet 0.31.0:

```
Running GHC WSGI on 0.0.0.0:80 with 4 workers and SCRIPT_NAME=

Error: class uri 'eventlet' invalid or not found: 

[Traceback (most recent call last):
  File "/venv/lib/python3.7/site-packages/gunicorn/util.py", line 135, in load_class
    mod = import_module('.'.join(components))
  File "/usr/local/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/venv/lib/python3.7/site-packages/gunicorn/workers/geventlet.py", line 23, in <module>
    from eventlet.wsgi import ALREADY_HANDLED as EVENTLET_ALREADY_HANDLED
ImportError: cannot import name 'ALREADY_HANDLED' from 'eventlet.wsgi' (/venv/lib/python3.7/site-packages/eventlet/wsgi.py)
]

```

trying upgrading `gunicorn` to `20.1.0` ...

docker/scripts/requirements.txt

@@ -1,5 +1,5 @@
 Paver==1.3.4
 psycopg2==2.7.5
-eventlet==0.31.0
-gunicorn==19.9.0
+eventlet==0.30.2
+gunicorn==20.1.0
 Werkzeug==0.16.1