Setup towncrier to generate release notes

newsfragments/README.md

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+This directory collects "newsfragments": short files that each contain
+a snippet of ReST-formatted text that will be added to the next
+release notes. This should be a description of aspects of the change
+(if any) that are relevant to users. (This contrasts with the
+commit message and PR description, which are a description of the change as
+relevant to people working on the code itself.)
+
+Each file should be named like `<ISSUE>.<TYPE>.rst`, where
+`<ISSUE>` is an issue numbers, and `<TYPE>` is one of:
+
+* `feature`
+* `bugfix`
+* `performance`
+* `doc`
+* `removal`
+* `misc`
+
+So for example: `123.feature.rst`, `456.bugfix.rst`
+
+If the PR fixes an issue, use that number here. If there is no issue,
+then open up the PR first and use the PR number for the newsfragment.
+
+Note that the `towncrier` tool will automatically
+reflow your text, so don't try to do any fancy formatting. Run
+ `towncrier --draft` to get a preview of what the release notes entry
+ will look like in the final release notes.

newsfragments/validate_files.py

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+#!/usr/bin/env python3
+
+# Towncrier silently ignores files that do not match the expected ending.
+# We use this script to ensure we catch these as errors in CI.
+
+import os
+import pathlib
+
+ALLOWED_EXTENSIONS = {
+    '.bugfix.rst',
+    '.doc.rst',
+    '.feature.rst',
+    '.misc.rst',
+    '.performance.rst',
+    '.removal.rst',
+}
+
+ALLOWED_FILES = {
+    'validate_files.py',
+    'README.md',
+}
+
+THIS_DIR = pathlib.Path(__file__).parent
+
+for fragment_file in THIS_DIR.iterdir():
+
+    if fragment_file.name in ALLOWED_FILES:
+        continue
+
+    full_extension = "".join(fragment_file.suffixes)
+    if full_extension not in ALLOWED_EXTENSIONS:
+        raise Exception(f"Unexpected file: {fragment_file}")