Requirements fix for urllib3 dependency mismatch

We're listing dependencies alphabetically. This causes pip to
resolve minio urllib3 dependency (which has an unspecified version)
before requests (which pins it to below 1.25), causing a version
mismatch.

This is a workaround that should be removed once
https://github.com/kennethreitz/requests/issues/5067 is resolved

requirements.txt

@@ -15,4 +15,10 @@ psycopg2-binary==2.8.2
 pyjexl==0.2.3
 python-memcached==1.59
 requests==2.21.0
+# urllib3 explicit version requirement. Pin it to below 1.25, as
+# requests doesn't work above that version (and minio would pull
+# in the newest version) Can be removed once the following issue
+# is resolved:
+# https://github.com/kennethreitz/requests/issues/5067
+urllib3>=1.21.1,<1.25
 uwsgi==2.0.18