pin gunicorn to git commit; bump eventlet

gunicorn doesn't pin eventlet, but functionally, gunicorn==20.1.0
depends on eventlet<=0.30.2 due to a change in eventlet. Gunicorn have
fixed this compat issue, however, haven't released it. By pinning to a
git commit, we're able to bump eventlet up to 0.33, thus solving a
security advisory. (Note that the security advisory didn't actually
impact us as it only affects websockets, however, it was noisy and
distracting).

Note - pip may have cached the old version of gunicorn. You may need to
run `pip install -r requirements.txt --no-cache-dir` to get the updated
version of gunicorn locally.

requirements.txt

@@ -67,14 +67,14 @@ click-repl==0.2.0
     # via celery
 colorama==0.4.3
     # via awscli
-dnspython==1.16.0
+dnspython==2.2.0
     # via eventlet
 docopt==0.6.2
     # via notifications-python-client
 docutils==0.15.2
     # via awscli
-eventlet==0.30.2
-    # via -r requirements.in
+eventlet==0.33.0
+    # via gunicorn
 flask==1.1.2
     # via
     #   -r requirements.in
@@ -106,7 +106,7 @@ greenlet==1.1.2
     # via
     #   eventlet
     #   sqlalchemy
-gunicorn==20.1.0
+gunicorn @ git+https://github.com/benoitc/gunicorn.git@1299ea9e967a61ae2edebe191082fd169b864c64
     # via -r requirements.in
 idna==3.3
     # via requests