Convert PR numbers in docs/change_log to clickable links (#4346)

Uses the sphinx include-read event to regex replace all occurrences of a PR number `(#X)` with a link `[(#X)](https://github.com/psf/black/pull/X)`.

docs/conf.py

@@ -13,10 +13,13 @@
 #
 
 import os
+import re
 import string
 from importlib.metadata import version
 from pathlib import Path
 
+from sphinx.application import Sphinx
+
 CURRENT_DIR = Path(__file__).parent
 
 
@@ -29,6 +32,36 @@ def make_pypi_svg(version: str) -> None:
         f.write(svg)
 
 
+def replace_pr_numbers_with_links(content: str) -> str:
+    """Replaces all PR numbers with the corresponding GitHub link."""
+
+    base_url = "https://github.com/psf/black/pull/"
+    pr_num_regex = re.compile(r"\(#(\d+)\)")
+
+    def num_to_link(match: re.Match[str]) -> str:
+        number = match.group(1)
+        url = f"{base_url}{number}"
+        return f"([#{number}]({url}))"
+
+    return pr_num_regex.sub(num_to_link, content)
+
+
+def handle_include_read(
+    app: Sphinx,
+    relative_path: Path,
+    parent_docname: str,
+    content: list[str],
+) -> None:
+    """Handler for the include-read sphinx event."""
+    if parent_docname == "change_log":
+        content[0] = replace_pr_numbers_with_links(content[0])
+
+
+def setup(app: Sphinx) -> None:
+    """Sets up a minimal sphinx extension."""
+    app.connect("include-read", handle_include_read)
+
+
 # Necessary so Click doesn't hit an encode error when called by
 # sphinxcontrib-programoutput on Windows.
 os.putenv("pythonioencoding", "utf-8")