chore: use new unittest.mock from standard library

[a-detiste]

https://github.com/testing-cabal/mock

"mock is now part of the Python standard library, available as unittest.mock in Python 3.3 onwards."

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [ x I have read the Contribution Guidelines and my PR follows them
• [ ] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

[tiwarishubham635]

I see some error here, can you check this?

[a-detiste]

Please merge https://github.com/twilio/twilio-python/pull/783 first and I will rebase if this can't be done automagicaly with github

[a-detiste]

So finally no #783 , I will trim this down

[tiwarishubham635]

So there will be some changes in this repo and some in generator repo. We can use #783 for this repo's changes.

[tiwarishubham635]

@a-detiste do you want to continue working on it?

[a-detiste]

Yes I can

[a-detiste]

Well it's done for me... only merge needed

[tiwarishubham635]

Please check the failing tests. if mock is part of python since 3.3 then it should work fine with 3.7 as well