Include co-authors in release announcement (pytest-dev#12795)

As noted in pytest-dev#12789, the `release.py` script did not consider `Co-authored-by` fields, and since we introduced the new backport bot, this problem became more apparent due to how the backport commit and PR are generated.

Previously, the list of authors produced by the script in the pytest-dev#12789 branch was:

```
* Avasam
* Bruno Oliveira
* Ronny Pfannschmidt
* Sviatoslav Sydorenko (Святослав Сидоренко)
```

With this script:

```
* Anthony Sottile
* Avasam
* Bruno Oliveira
* Christian Clauss
* Eugene Mwangi
* Florian Bruhin
* GTowers1
* Nauman Ahmed
* Pierre Sassoulas
* Reagan Lee
* Ronny Pfannschmidt
* Stefaan Lippens
* Sviatoslav Sydorenko (Святослав Сидоренко)
* dongfangtianyu
```

scripts/release.py

@@ -6,6 +6,7 @@
 import argparse
 import os
 from pathlib import Path
+import re
 from subprocess import call
 from subprocess import check_call
 from subprocess import check_output
@@ -16,17 +17,27 @@
 
 def announce(version: str, template_name: str, doc_version: str) -> None:
     """Generates a new release announcement entry in the docs."""
-    # Get our list of authors
+    # Get our list of authors and co-authors.
     stdout = check_output(["git", "describe", "--abbrev=0", "--tags"], encoding="UTF-8")
     last_version = stdout.strip()
+    rev_range = f"{last_version}..HEAD"
 
-    stdout = check_output(
-        ["git", "log", f"{last_version}..HEAD", "--format=%aN"], encoding="UTF-8"
+    authors = check_output(
+        ["git", "log", rev_range, "--format=%aN"], encoding="UTF-8"
+    ).splitlines()
+
+    co_authors_output = check_output(
+        ["git", "log", rev_range, "--format=%(trailers:key=Co-authored-by) "],
+        encoding="UTF-8",
     )
+    co_authors: list[str] = []
+    for co_author_line in co_authors_output.splitlines():
+        if m := re.search(r"Co-authored-by: (.+?)<", co_author_line):
+            co_authors.append(m.group(1).strip())
 
     contributors = {
         name
-        for name in stdout.splitlines()
+        for name in authors + co_authors
         if not name.endswith("[bot]") and name != "pytest bot"
     }