Add description to Tox's environments

[kianmeng]

$ tox -av
...
default environments:
py36  -> testing against python3.6
py37  -> testing against python3.7
py38  -> testing against python3.8
py39  -> testing against python3.9
pypy3 -> testing against pypy3

[AzisK]

Thanks for working on this. Could you update the code with the newest from the master branch?

[coveralls]

coverage: 98.43%. remained the same when pulling d9b9020 on kianmeng:add-desc-to-tox-environments into 12a8f33 on kayak:master.

[kianmeng]

🥳 🥳 🥳 🥳 🥳