Drop Python 3.6, default to running on 3.10

Python 3.6 is well beyond end-of-life (December 2021). While 3.7
is still supported, it doesn't have long left (mid 2023). We should
therefore still support using it, but there's no compelling reason
to default to it.

.circleci/config.yml

@@ -15,7 +15,7 @@ references:
     PG_DB: routemaster
     PG_USER: routemaster
     PG_PASS: routemaster
-    TOXENV: py36
+    TOXENV: py37
     COVERALLS: true
 
   # Set up a cached virtualenv in which to install our CI dependencies
@@ -96,17 +96,6 @@ references:
             coveralls
 
 jobs:
-  test-36:
-    <<: *test-template
-    docker:
-      - image: cimg/python:3.6
-        environment:
-          <<: *steps-environment
-          TOXENV: py36
-          COVERALLS: true
-
-      - *postgres-container
-
   test-37:
     <<: *test-template
     docker:
@@ -154,7 +143,7 @@ jobs:
   lint:
     working_directory: ~/routemaster
     docker:
-      - image: cimg/python:3.6
+      - image: cimg/python:3.7
     steps:
       - checkout
 
@@ -181,7 +170,7 @@ jobs:
   typecheck:
     working_directory: ~/routemaster
     docker:
-      - image: cimg/python:3.6
+      - image: cimg/python:3.7
     steps:
       - checkout
 
@@ -208,7 +197,7 @@ jobs:
   release:
     working_directory: ~/routemaster
     docker:
-      - image: cimg/python:3.6
+      - image: cimg/python:3.7
     steps:
       - checkout
 
@@ -275,10 +264,6 @@ workflows:
   version: 2
   build-release:
     jobs:
-      - test-36:
-          filters:
-            tags:
-              only: /.*/
       - test-37:
           filters:
             tags:
@@ -305,7 +290,6 @@ workflows:
               only: /.*/
       - release:
           requires:
-            - test-36
             - test-37
             - test-38
             - test-39

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.6-stretch
+FROM python:3.10-bullseye
 
 ENV PYTHONUNBUFFERED 1
 

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py37,py38,py39,py310,mypy,lint
+envlist = py37,py38,py39,py310,mypy,lint
 
 [testenv]
 deps =