Consolidate Python dependency definitions into a Pipfile-backed packaging flow

[Copilot]

This change removes the split dependency management across multiple requirements files and makes Pipfile the single source of truth for repository dependencies. Packaging remains pip-compatible, so installs from PyPI and from source continue to work through standard package metadata.

• Single dependency source

  • adds a root Pipfile for runtime and development dependencies
  • removes requirements.txt and test-requirements.txt
  • keeps dependency intent centralized instead of duplicating versions across files

• Packaging metadata

  • updates setup.py to derive install_requires, tests_require, and a test extra from Pipfile
  • reads package version from source without importing the package during isolated builds
  • caches parsed Pipfile metadata to avoid repeated TOML reads

• Build compatibility

  • adds pyproject.toml build requirements so TOML parsing works in isolated builds, including Python versions without stdlib tomllib
  • preserves pip install ipwb / pip install . behavior by continuing to emit standard setuptools metadata

• CI and contributor workflow

  • updates workflows to install test dependencies via package metadata (pip install '.[test]') rather than deleted requirements files
  • documents the Pipenv-based developer setup in README.md

• Focused metadata coverage

  • adds targeted tests that assert setup metadata is generated from Pipfile

Example of the new CI/source install path:

pip install '.[test]'