templates: add Python bump checklist

Imported verbatim from wiki HowToRelease.

Author: bgilbert

.github/ISSUE_TEMPLATE/python-bump.md

@@ -0,0 +1,11 @@
+# Adding wheels for a new Python release
+
+- [ ] [Add two new build matrix groups](https://ci.appveyor.com/project/OpenSlide/openslide-python/settings/environment) to AppVeyor job with appropriate PYTHON variables
+- [ ] [Select the original OpenSlide Python release build](https://ci.appveyor.com/project/OpenSlide/openslide-python/history) in AppVeyor, then click "Rebuild Commit"
+- [ ] In OpenSlide Python checkout, `git checkout v<version> && git clean -dxf && mkdir dist`
+- [ ] Download wheels _from new build jobs only_ into `dist` directory
+- [ ] `twine upload dist/*`
+- [ ] Upload new wheels to [GitHub release](https://github.com/openslide/openslide-python/releases)
+- [ ] [Download new AppVeyor YAML](https://ci.appveyor.com/project/OpenSlide/openslide-python/settings/yaml) and commit it to `openslide-automation/appveyor/openslide-python.yml`
+- [ ] Update MacPorts package
+- [ ] Update website: Python 3 versions in `download/index.markdown`