Release 1.2.0

[bgilbert]

OpenSlide Python release process

• Update CHANGELOG.md and version in openslide/_version.py
• Create and push signed tag
• git clean -dxf && mkdir dist
• Find the workflow run for the tag; download its docs and wheels artifacts
• unzip /path/to/downloaded/openslide-python-wheels.zip && mv openslide-python-wheels-*/* dist/
• python setup.py sdist
• twine upload dist/*
• Recompress tarball with xz
• Attach release notes to GitHub release; upload tarballs and wheels
• cd into website checkout; rm -r api/python && unzip /path/to/downloaded/openslide-python-docs.zip && mv openslide-python-docs-* api/python
• Update website: _data/releases.yaml, _includes/news.md
• Send mail to -announce and -users
• Update Fedora package
• Update MacPorts package