Add PIL Image import for type hints under TYPE_CHECKING

Resolves the Image.Image forward reference in load_pdf's return type
without adding PIL as a runtime import.

Based on: Layout-Parser#174 (kairi003)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: IgorTavcar

src/layoutparser/io/pdf.py

@@ -13,11 +13,14 @@
 # limitations under the License.
 
 from io import BytesIO
-from typing import BinaryIO, List, Union, Optional, Dict, Tuple
+from typing import BinaryIO, List, Union, Optional, Dict, Tuple, TYPE_CHECKING
 
 import pdfplumber
 import pandas as pd
 
+if TYPE_CHECKING:
+    from PIL import Image
+
 from ..elements import Layout
 from .basic import load_dataframe