refactor: typehints and imports compatible to Python3.7

[khubaibalam2000]

Issue #, if available:
This Pull Request resolve the issue mentioned in #627.

Description of changes:
Removed the imports as it is already supported in Python v3.7.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ShubhamChaturvedi7]

@khubaibalam2000 Could you use the semantically correct commit name / PR title?

[ShubhamChaturvedi7]

Also, a quick search shows that typehints and imports are scattered all over the place and we should fix them all: https://github.com/search?q=repo%3Aaws%2Faws-dynamodb-encryption-python+%22from+typing+import+Dict%22&type=code

[khubaibalam2000]

Sure, in the next commit, I am fixing these imports.

[texastony]

@khubaibalam2000 You recent commits,
probably b4e40e3,
have broken the library.

You should be able to view the logs of the GitHub actions to examine
what tests failed.

You can look at the GitHub Workflow File
in the repo for guidance on running the CI locally.

[khubaibalam2000]

@texastony Thank you for your guidance. Now, I am debugging and viewing the CI logs locally and fixing them. Will provide you with unbreakable commit soon.

[ShubhamChaturvedi7]

@khubaibalam2000 There are still a lot of CIs failing. Please run them locally so that you can quickly iterate over the issues and fix them. Let us know if you need any help.

[khubaibalam2000]

Yes, as per your suggestion, I setup the CI locally and debug the code and found some errors, but now all the error are fixed. In my latest commit 7ac6b60, all CIs are passing. Thank you for yours guidance.

[khubaibalam2000]

All the checks are now passing. Is there any work left to do about the issue? or is it done.

[lucasmcdonald3]

Thank you for your contribution!

[khubaibalam2000]

Thank you for your contribution!

Welcome. Good to work with this repo. Going to work in the future issues and PRs too.