remove then-sleep dev dependency

[Uzlopak]

Why use a package if we can use native promises and it is basically a three liner.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[voxpelli]

And from Node.js v15 this is even built into Node.js: https://nodejs.org/api/timers.html#timerspromisessettimeoutdelay-value-options