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/pdf.py

@@ -80,14 +80,14 @@ class BasePDFLoader(BaseLoader, ABC):
         clean up the temporary file after completion.
     """
 
-    def __init__(self, file_path: str, *, headers: Optional[Dict] = None):
+    def __init__(self, file_path: Union[str, Path], *, headers: Optional[Dict] = None):
         """Initialize with a file path.
 
         Args:
             file_path: Either a local, S3 or web path to a PDF file.
             headers: Headers to use for GET request to download a file from a web path.
         """
-        self.file_path = file_path
+        self.file_path = str(file_path)
         self.web_path = None
         self.headers = headers
         if "~" in self.file_path:
@@ -226,7 +226,7 @@ class PyPDFDirectoryLoader(BaseLoader):
 
     def __init__(
         self,
-        path: str,
+        path: Union[str, Path],
         glob: str = "**/[!.]*.pdf",
         silent_errors: bool = False,
         load_hidden: bool = False,