Upgrade syntax of setup.py to python 3.8+

[tanmaysinghal98]

This is a fix for #609
Please take a look and let me know.

Checked with following command

find . -iname '*.py' | grep -Ev 'vendor|example|doc|tools|sphinx' | xargs -P4 -I{} python3.8 -Wall -m py_compile {}

No deprecation warning coming.

[coveralls]

Coverage increased (+0.1%) to 92.918% when pulling 6ddaba2 on tanmaysinghal98:master into 6ccc5a9 on mozilla:master.

[whimboo]

I'm very sorry that it took so long to review. I now had a look at and compared to the other PRs this is the right solution. Thanks a lot!