PyMuPDF-2.1 released

Wheels for Windows, Linux and MacOS, and the sdist, are available on
https://pypi.org and can be installed in the usual way, for example:

python -m pip install --upgrade pymupdf

Changes in version 2.1

Fixed issues:

• Fixed #5196
• Fixed #5214
• Fixed #5216
• Fixed #5220

Other:

• Improve upgrade from 1.x to 2.x via pymupdf4llm or pymupdf_layout.
• Reduced startup time using lazy internal import of large modules.
• Retrospectively added date of 2.0 release.
• Retrospectively marked #5115 as fixed in 2.0.
• Use MuPDF-1.28.6.