Use walrus operator

Co-authored-by: Andrew Murray <3112309+radarhere@users.noreply.github.com>

Author: hugovk

.github/compare-dist-sizes.py

@@ -50,12 +50,10 @@
 
 def key_for(filename: str) -> str | None:
     """Return a version-independent identifier for a dist file."""
-    m = WHEEL_RE.match(filename)
-    if m:
+    if m := WHEEL_RE.match(filename):
         build = f"-{m['build']}" if m["build"] else ""
         return f"wheel:{build}-{m['python']}-{m['abi']}-{m['platform']}"
-    m = SDIST_RE.match(filename)
-    if m:
+    if m := SDIST_RE.match(filename):
         return "sdist"
     return None
 
@@ -275,8 +273,7 @@ def main() -> int:
 
     print(render_table(baseline_label, baseline_sizes, local_sizes, markdown=False))
 
-    summary_path = os.environ.get("GITHUB_STEP_SUMMARY")
-    if summary_path:
+    if summary_path := os.environ.get("GITHUB_STEP_SUMMARY"):
         with open(summary_path, "a", encoding="utf-8") as f:
             f.write(
                 render_table(baseline_label, baseline_sizes, local_sizes, markdown=True)