[Feature] Multithreading feature in python3 for corresponding python2 files

The master branch is maintained for python2 whereas the feature/python3.8 branch is maintained for python3. Multithreading feature is available in python2 as of now.

The multithreading feature has been used in the project to improve efficiency of the scanning process. Checkout 'Need for Multithreading' section in README.md for better understanding.

Create a PR in feature/python3.8 branch for the multithreading feature in python3.

The file correspondence in python2 - python3(master - feature/python3.8) can be understood as follows:

1. src/single/scanner.py - scanner.py ( Port scanning feature without multithreading )
2. src/multi/scanner_thread.py - (to be written in py3) Split processing function for multithreading.
3. src/mainScanner.py - (to be written in py3) Port Scanning feature using multithreading.
4. src/ipscanner - (to be written in py3) IP Scanning feature using multithreading.

Follow the same directory structure as in master.