Fix concurrency issue

When multiple threads are running, they may both
cleanup the cache directory at the same time and
one thread will provoke FileNotFound errors in the
other thread.

src/markdown_katex/wrapper.py

@@ -284,10 +284,17 @@ def tex2html(tex: str, options: MaybeOptions = None) -> str:
 def _cleanup_cache_dir() -> None:
     min_mtime = time.time() - 24 * 60 * 60
     for fpath in CACHE_DIR.iterdir():
-        if fpath.is_file():
+        try:
+            if not fpath.is_file():
+                continue
+
             mtime = fpath.stat().st_mtime
-            if mtime < min_mtime:
-                fpath.unlink()
+            if mtime > min_mtime:
+                continue
+
+            fpath.unlink()
+        except FileNotFoundError:
+            pass  # concurrent thread deleted file before we did
 
 
 # NOTE: in order to not have to update the code