[pipreqs/pipreqs.py] Use `codec` to get an `open` with `encoding` arg…

[SamuelMarks]

…ument ; fix __version__ read (import cannot work) ; don't crash on UnicodeDecodeError

[jonas-eschle]

Thanks for the PR! Can you explain a bit more what is going on?

[SamuelMarks]

I wrote this 2.5 years ago 🤷‍♂️

Your codebase was probably giving me an error and I figured out how to resolve it so sent you a PR

[jonas-eschle]

Yes, I am aware, I just took over the maintenance, has been a bit undermaintained. I'll look through it, thanks a lot for the PR!