update classifiers

[anukaal]

use python new versions 3.7, 3.8, 3.9 and 3.10

[jlyang1990]

Hi @anukaal , could you explain more on this issue? We actually tested this package on multiple Python new versions (higher than 3.6), and we found this package consistently worked well. Thanks!

[jlyang1990]

Added Python new versions 3.7, 3.8, 3.9 and 3.10 into setup.py.