gh-155212: Check for a directory before opening the file

Opening a directory fails with IsADirectoryError on Unix, but with
PermissionError on Windows.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Tools/clinic/libclinic/cli.py

@@ -69,12 +69,12 @@ def parse_file(
     except KeyError:
         raise ClinicError(f"Can't identify file type for file {filename!r}")
 
-    try:
-        with open(filename, encoding="utf-8") as f:
-            raw = f.read()
-    except IsADirectoryError:
+    if os.path.isdir(filename):
         raise ClinicError(f"Can't read file {filename!r}: it is a directory")
 
+    with open(filename, encoding="utf-8") as f:
+        raw = f.read()
+
     # exit quickly if there are no clinic markers in the file
     find_start_re = BlockParser("", language).find_start_re
     if not find_start_re.search(raw):