build: Prepare release

RELEASE.md

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+Release type: minor
+
+- Allow changefreq and priority to be set by page/article metadata ([#30](https://github.com/pelican-plugins/sitemap/pull/30) by [dgaylo](https://github.com/dgaylo))
+- Fix error message for invalid changefreq value ([#31](https://github.com/pelican-plugins/sitemap/pull/31) by [dgaylo](https://github.com/dgaylo))
+- Fix error in tests ([#37](https://github.com/pelican-plugins/sitemap/pull/37) by [projectgus](https://github.com/projectgus))
+- Support Python 3.9 – 3.13 ([#38](https://github.com/pelican-plugins/sitemap/pull/38) by [justinmayer](https://github.com/justinmayer))
+- Pin Blinker temporarily to 1.7 until upstream fix is released ([#35](https://github.com/pelican-plugins/sitemap/issues/35) by [justinmayer](https://github.com/justinmayer))