VOL-2241: Python 3 migration

 - update and remove unused requirements
 - use python/debian buster base image (includes pip)

Change-Id: I9d55f1c6ed87bbb1cee8e86fe66eb29797680f80
diff --git a/requirements.txt b/requirements.txt
index 02bb3d7..11cf681 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,11 +4,10 @@
 cmd2==0.7.0
 colorama==0.3.9
 confluent-kafka==0.11.5
-cython==0.24.1
+cython==0.29.14
 decorator==4.1.2
 docker-py==1.10.6
 fluent-logger==0.6.0
-grpc==0.3.post19
 grpcio==1.16.0
 grpcio-tools==1.16.0
 hash_ring==1.3.1
@@ -32,23 +31,23 @@
 pyOpenSSL==17.3.0
 PyYAML==3.12
 requests==2.18.4
-scapy==2.3.3
-service-identity==17.0.0
+scapy==2.4.3
+service-identity==18.1.0
 simplejson==3.12.0
 jsonschema==2.6.0
 six==1.12.0
-structlog==17.2.0
+structlog==19.2.0
 termcolor==1.1.0
 transitions==0.6.4
 treq==17.8.0
-Twisted==18.7.0
+Twisted==19.10.0
 txaioetcd==0.3.0
 urllib3==1.22
 pyang==1.7.3
-lxml==3.6.4
+lxml==4.4.1
 nosexcover==1.0.11
 zmq==0.0.0
-pyzmq==16.0.3
+pyzmq==18.1.1
 txZMQ==0.8.0
 ncclient==0.5.3
 xmltodict==0.11.0