Opened up Makefile to accept newer versions of Python than 3.8

[simonprickett]

Closes #67 - adds Python 3.9, 3.10 support in Makefile check.

Test run output:

$ make test
. env/bin/activate; pytest "-s"
============================ test session starts ============================
platform darwin -- Python 3.9.5, pytest-5.4.1, py-1.10.0, pluggy-0.13.1
rootdir: /Users/simonprickett/tmp/ru102py, inifile: pytest.ini
plugins: dotenv-0.5.2
collected 67 items

tests/test_hello.py .
tests/test_quiz_questions.py .......
tests/test_redis_basics.py ....
tests/test_streams.py ...
tests/test_transactions.py ..
tests/api/test_capacity_api.py ..
tests/api/test_meter_reading_api.py ssss
tests/api/test_metrics_api.py sss
tests/api/test_site_api.py .s
tests/api/test_site_geo_api.py ...
tests/dao/redis/test_capacity_report.py ..s
tests/dao/redis/test_feed.py s
tests/dao/redis/test_fixed_rate_limiter.py ..
tests/dao/redis/test_meter_reading.py .
tests/dao/redis/test_metric.py sss
tests/dao/redis/test_metric_timeseries.py ...
tests/dao/redis/test_schema.py ...
tests/dao/redis/test_site.py ....s
tests/dao/redis/test_site_geo.py ......s
tests/dao/redis/test_site_stats.py .
tests/dao/redis/test_sliding_window_rate_limiter.py sss
tests/models/test_models.py ..
tests/scripts/test_update_if_lowest.py ..

====================== 49 passed, 18 skipped in 34.47s ======================