Fix setuptools>=68 build error with pyproject.toml and MANIFEST.in

[oxqnd]

This PR resolves build issues caused by setuptools>=68 as reported in dogecoin/dogecoin#3583.

🔧 Fix Summary

• Added pyproject.toml to enable modern (PEP 517) builds
• Added MANIFEST.in to ensure scrypt.h is included in source distribution

✅ Verified On

• Ubuntu focal (via Docker)
• Python 3.8.2
• pip 25.0.1
• setuptools 75.3.2

Build and install succeeded using:

FROM ubuntu:focal

RUN apt update && apt install -y python3 python3-pip python3-setuptools curl build-essential
WORKDIR /build
COPY . .
RUN python3 -m pip install --upgrade pip
RUN python3 -m pip install dist/ltc_scrypt-1.0.99.tar.gz

[patricklodder]

CI errors are currently out of scope for this PR - I'll make a separate change for these.

[oxqnd]

Hi @patricklodder , just a quick heads-up — the bionic-py36-gcc7 job has been stuck in queue for over 17 hours now.
Since you mentioned CI errors are out of scope, just let me know if you'd prefer I force-push to retrigger or leave it as-is.

Thanks again!

[patricklodder]

I cancelled it manually. Don't worry about it for now.

[oxqnd]

@patricklodder Thanks for the update, appreciate it! I’ll leave it in your hands from here. 🙌

[oxqnd]

Hi @patricklodder, just checking in to see if there's any update on this.
Totally understand if things are busy — just didn't want it to slip through the cracks 😄