Give description to tox environments

[kianmeng]

$ tox -av
...
default environments:
py27   -> run test on python2.7
py34   -> run test on python3.4
py35   -> run test on python3.5
py36   -> run test on python3.6
py37   -> run test on python3.7
py38   -> run test on python3.8
py39   -> run test on python3.9
py310  -> run test on python3.10
pypy   -> run test on pypy
pypy3  -> run test on pypy3
docs   -> mimic readthedocs build
readme -> mimic pypi validation of readme/rst files
black  -> show diff of code format