Allow gunicorn to start with Python 3.10

Without upgrading gunicorn (and, by extension, eventlet and greenlet),
gunicorn won't start, complaining of a syntax error inside gunicorn. I
didn't catch this earlier in this branch because I wasn't running in
production mode.

requirements.txt

@@ -1,9 +1,9 @@
 django~=5.0
 redis==2.10.6
 boto==2.49.0
-gunicorn==19.3.0
+gunicorn~=21.2
 pyScss~=1.4
-eventlet~=0.20.1
+eventlet~=0.36.1
 whitenoise~=4.1.4
-greenlet~=1.1.3
+greenlet~=3.0
 tzdata