Add requirements.txt and test_files to source dist

[freddyaboulton]

Description

Fixes #1812

How to test

1. Clone this branch
2. python setup.py sdist
3. In a fresh env, pip install dist/gradio-3.0.26.tar.gz should install gradio + all dependencies
4. Unpack the source distribution. test/test_files and test/requirements.txt should be included.
   

Checklist:

• I have performed a self-review of my own code
• My code follows the style guidelines of this project
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[freddyaboulton]

@pbsds can you let me know if you need the test requirements?

[pbsds]

I do not personally, but their inclusion is at best a kilobyte

[freddyaboulton]

Awesome, thank you!