Fix PyPI README rendering metadata

[marko1olo]

Fixes #1317.

This fixes the PyPI README rendering metadata and uses the canonical raw GitHub URL for the Quantum AI logo image.

Changes:

• Declares long_description_content_type='text/markdown' so PyPI renders the README as Markdown/HTML instead of plain text.
• Replaces the refs/heads/main raw image URL with the canonical /main/ raw URL.

Validation:

• python -m build --sdist --wheel from isolated venv C:\hades\oss\.venvs\openfermion-1317
• python -m twine check dist\*
• Verified generated metadata includes Description-Content-Type: text/markdown

Note: build emitted an existing setuptools license-classifier deprecation warning unrelated to this change.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request updates an image source URL in README.md and specifies the long description content type as markdown in setup.py. The reviewer pointed out that setting the content type to markdown causes a duplicate title rendering on PyPI due to a prepended RST-style header in setup.py, and suggested reading the README file directly without prepending the redundant header.

[gemini-code-assist]

Setting long_description_content_type to 'text/markdown' causes the prepended RST-style header on lines 23-25 (long_description = '===========\n' + 'OpenFermion\n' + '===========\n\n') to be parsed as a Markdown H1 header. Since README.md already contains a # OpenFermion header, this results in a duplicate title rendering on PyPI.

To fix this, please update lines 23-25 to read the file directly without prepending the redundant header:

with open('README.md', 'r', encoding='utf-8') as readme:
    long_description = readme.read()