Update calculator.py

[Sanketh149]

I added quit button to get out of calculator whenever needed

Description

I just added a quit button to come out from the calculator application because it looks more user interaction friendly

Type of change

Choosing one or more options from the following as per the nature of your Pull request.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation Update

Checklist:

Please tick all the boxes that are fulfilled by your Pull Request.

• I have named my files and folder, according to this project's guidelines.
• My code follows the style guidelines of this project.
• My Pull Request has a descriptive title. (not a vague title like Update index.md)
• I have commented on my code, particularly in hard-to-understand areas.
• I have created a helpful and easy to understand README.md, according to the given README_TEMPLATE.
• I have included a requirements.txt file (if external libraries are required.)
• My changes do not produce any warnings.
• I have added a working sample/screenshot of the script.