fix(dependencies): pin pdfminer.six to 20231228 for compatibility

requirements.txt

@@ -7,5 +7,5 @@ numpy>=1.21.6
 fast-langdetect>=0.2.1
 wordninja>=2.0.0
 scikit-learn>=1.0.2
-pdfminer.six>=20231228
+pdfminer.six==20231228
 # The requirements.txt must ensure that only necessary external dependencies are introduced. If there are new dependencies to add, please contact the project administrator.