GitLab

__Description__

Update Python setup for require packages.

__Major Revisions__
* downgrade requests version to be compatible with python 3.6, add corresponding pipeline for 3.6
* add extra entry in extras_require for nested packages
* update `pip install` contents accordingly

 Add config and docs for development experience.

__Major Revision__
- Add settings and extensions config for VSCode.
- Add devcontainer config for Codespaces.
- Update document accordingly.