Switch to python 3.6 as testenv, and add backport dependencies.

Author: Diego Ardila

.circleci/config.yml

@@ -4,7 +4,7 @@ version: 2.1
 jobs:
   build_test:
     docker:
-      - image: python:3.9-slim-buster
+      - image: python:3.6-slim-buster
     resource_class: small
     steps:
       - checkout # checkout source code to working directory

pyproject.toml

@@ -39,6 +39,7 @@ tqdm = "^4.60.0"
 boto3 = "^1.17.53"
 mypy = "^0.812"
 coverage = "^5.5"
+dataclasses = { version = "^0.7", python = "^3.6.1, <3.7" }
 
 [tool.poetry.dev-dependencies]
 poetry = "^1.1.5"