[feature-request] Use robot to update requirements.txt

[bebound]

requirements.txt is introduced in #9785. It should be the result of pip freeze.
However, as time goes by, these files are edit manually when we bump package. Some dependencies are out-of-date and may have version conflict (see #28017).

I think we use a robot to improve this:

1. Developers only need to modify setup.py.
2. The robot creates requirements.txt using pip freeze and commits it to the pull request.
3. (The robot may also periodically update requirements.txt.)

[yonzhan]

Thank you for opening this issue, we will look into it.

[bebound]

I tried to use dependabot to do this. But it does not meet the requirement.

1. It does not follow the constraint in setup.py and tries to update all package into latest version.
   https://github.com/bebound/azure-cli-dependa/pulls
   

2. pip-compile mode is close to our need, but we must create a requirements.in to tell dependabot to follow this file. Because it does not support setup.py file for now. However, we need 3 different requirements.txt. And I don't know how to tell dependabot to follow the requirements.in file.

PS: We can copy the dependency in setup.py to requirements.in and use this command to generate requirements.txt: pip-compile requirements.in --output-file requirements.txt --upgrade