Update version requirements (#642)

- Legacy opcodes are supported till MongoDB 5.0.
- Python 3.6 is EOL.
- Write versions without explicit string quotes is dangerous in YAML.

---------

Co-authored-by: Soha Jin <[email protected]>

Author: twd2

.github/workflows/test.yml

@@ -74,14 +74,14 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: ['3.7', '3.8', '3.9']
     services:
       rabbitmq:
         image: rabbitmq
         ports:
           - "5672:5672"
       mongodb:
-        image: mongo
+        image: mongo:5.0
         ports:
           - "27017:27017"
     steps:

README.md

@@ -43,8 +43,8 @@ There is pre-built docker package at `vijos/vj4`. This is maintained by [@moesoh
 
 #### Prerequisites
 
-* [Python 3.6+](https://www.python.org/downloads/)
-* [MongoDB 3.0+, <3.6](https://docs.mongodb.org/manual/installation/)
+* [Python 3.7+, <=3.9](https://www.python.org/downloads/)
+* [MongoDB 3.0+, <=5.0](https://www.mongodb.com/docs/v5.0/installation/)
 * [Node.js 10.0+](https://nodejs.org/en/download/package-manager/)
 * [RabbitMQ](http://www.rabbitmq.com/)