Add missing requirements.txt

[sunpoet]

Fixes #266

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• Code Change
• Example/Test Code Change

Validation

• Have you ran tox?
• Do the tests pass?

[CLAassistant]

All committers have signed the CLA.

[sunpoet]

The CLA does not work at all. If you click "Contributor License Agreement", it just open a new page with the same URL and content.

[greg-db]

Thanks for submitting this! I'll ask the team to review this.

Also thanks for letting us know about the issue with that CLA page. I'll ask the team to fix that up.