Migrate to a uv project

[caspervonb]

Moves to uv as the development tool, amongst other things (easier to work with first and foremost) it also gives us wheels when publishing.

Closes #9

[Copilot]

Pull Request Overview

This PR migrates the project from Pipenv to uv for dependency management and reorganizes the package structure to use a modern src-layout. The main changes include:

• Moving the nkeys module from the root to src/nkeys/ directory
• Replacing Pipenv (Pipfile/Pipfile.lock) with uv-based dependency management
• Updating Python version requirements from >=3.6 to >=3.7
• Consolidating development dependencies into pyproject.toml's optional-dependencies
• Updating build tooling references throughout the project

Reviewed Changes

Copilot reviewed 4 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/nkeys/init.py	Added complete nkeys implementation with cryptographic signing/verification functionality
setup.py	Removed deprecated setup.py in favor of pyproject.toml configuration
pyproject.toml	Updated minimum Python version to 3.7, added dev dependencies, configured for src-layout, enhanced mypy strictness
Pipfile.lock	Removed Pipenv lock file as part of migration to uv
Pipfile	Removed Pipfile as part of migration to uv
Makefile	Updated all commands to use uv run instead of pipenv run
.gitignore	Added uv.lock to ignored files

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