Declare requests as an install dependency

[szx19970521]

Fixes #2.

The published package imports requests at module import time, but the package metadata only installs habanero and bibtexparser. With current habanero, requests is no longer pulled transitively, so a clean install fails before users can instantiate ORCID2Markdown.

Reproduced with:

python -m venv D:\Temp\orcid2markdown-pip-repro-codex
D:\Temp\orcid2markdown-pip-repro-codex\Scripts\python.exe -m pip install orcid2markdown
D:\Temp\orcid2markdown-pip-repro-codex\Scripts\python.exe -c "from orcid2markdown import ORCID2Markdown"

Before this change:

ModuleNotFoundError: No module named 'requests'

This PR adds the packaging files that are already present in the PyPI sdist, declares requests in install_requires, and also updates/adds requirements files for source users.

Verified from this branch with a clean venv:

python -m pip install D:\Temp\orcid2markdown-fix-verify-codex\ORCID2Markdown-codex-declare-requests-dependency-20260608
python -c "from orcid2markdown import ORCID2Markdown; print('IMPORT_OK', ORCID2Markdown.__name__)"

Result:

IMPORT_OK ORCID2Markdown