docs: Add a snippet for python 3.5 deprecation [skip ci]

docs/markdown/snippets/python35-eol.md

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+## Python 3.5 support will be dropped in the next release
+
+The final [Python 3.5 release was 3.5.10 in September](https://www.python.org/dev/peps/pep-0478/#id4).
+This release series is now End-of-Life (EOL). The only LTS distribution that
+still only ships Python 3.5 is Ubuntu 16.04, which will be
+[EOL in April 2021](https://ubuntu.com/about/release-cycle).
+
+Python 3.6 has numerous features that we find useful such as improved support
+for the `typing` module, f-string support, and better integration with the
+`pathlib` module.
+
+As a result, we will begin requiring Python 3.6 or newer in Meson 0.57, which
+is the next release. Starting with Meson 0.56, we now print a `NOTICE:` when
+a `meson` command is run on Python 3.5 to inform users about this. This notice
+has also been backported into the 0.55.2 stable release.