community: better support of pathlib paths in document loaders (#18396)

So this arose from the
https://github.com/langchain-ai/langchain/pull/18397 problem of document
loaders not supporting `pathlib.Path`.

This pull request provides more uniform support for Path as an argument.
The core ideas for this upgrade: 
- if there is a local file path used as an argument, it should be
supported as `pathlib.Path`
- if there are some external calls that may or may not support Pathlib,
the argument is immidiately converted to `str`
- if there `self.file_path` is used in a way that it allows for it to
stay pathlib without conversion, is is only converted for the metadata.

Twitter handle: https://twitter.com/mwmajewsk

libs/community/langchain_community/document_loaders/email.py

@@ -1,5 +1,6 @@
 import os
-from typing import Any, Iterator, List
+from pathlib import Path
+from typing import Any, Iterator, List, Union
 
 from langchain_core.documents import Document
 
@@ -41,7 +42,10 @@ class UnstructuredEmailLoader(UnstructuredFileLoader):
     """
 
     def __init__(
-        self, file_path: str, mode: str = "single", **unstructured_kwargs: Any
+        self,
+        file_path: Union[str, Path],
+        mode: str = "single",
+        **unstructured_kwargs: Any,
     ):
         process_attachments = unstructured_kwargs.get("process_attachments")
         attachment_partitioner = unstructured_kwargs.get("attachment_partitioner")
@@ -79,17 +83,17 @@ class OutlookMessageLoader(BaseLoader):
     https://github.com/TeamMsgExtractor/msg-extractor
     """
 
-    def __init__(self, file_path: str):
+    def __init__(self, file_path: Union[str, Path]):
         """Initialize with a file path.
 
         Args:
             file_path: The path to the Outlook Message file.
         """
 
-        self.file_path = file_path
+        self.file_path = str(file_path)
 
         if not os.path.isfile(self.file_path):
-            raise ValueError("File path %s is not a valid file" % self.file_path)
+            raise ValueError(f"File path {self.file_path} is not a valid file")
 
         try:
             import extract_msg  # noqa:F401