Add missing requirements.txt

[rhui-dbx]

Can't seem to merge this: #335

Replicating the fix and trying to merge it

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?