Optimization of python solution_3 (using numpy)

[Volbla]

Description

Making the sieve faster by working closer with numpy's datatype, and by performing fewer calculations in plain python. I measure a 15% - 20% speed increase.

Renamed two local variables for clarity (v -> bit; step -> prime).

Contributing requirements

• [x] I read the contribution guidelines in CONTRIBUTING.md.
• [x] I placed my solution in the correct solution folder.
• [ ] I added a README.md with the right badge(s).
• [ ] I added a Dockerfile that builds and runs my solution.
• [x] I selected drag-race as the target branch.
• [x] All code herein is licensed compatible with BSD-3.

[rbergen]

@Volbla Thank you for your submission.

@emillynge I'm interested in your feedback on this proposed change to your solution, if you'd like to provide any?

[rbergen]

I can confirm a speed increase, but on my system it's close to 10%. That's still an improvement, so I'll merge this after CI completes successfully.