feat: add support for `.ppt` files in `UnstructuredPowerPointLoader` (#1124)

###  Summary

Adds support for older `.ppt` file in the PowerPoint loader. 

### Testing

The following should work on `unstructured==0.4.11` using the example
docs from the `unstructured` repo.

```python
from langchain.document_loaders import UnstructuredPowerPointLoader

filename = "../unstructured/example-docs/fake-power-point.pptx"
loader = UnstructuredPowerPointLoader(filename)
loader.load()

filename = "../unstructured/example-docs/fake-power-point.ppt"
loader = UnstructuredPowerPointLoader(filename)
loader.load()
```

Now downgrade `unstructured` to version `0.4.10`. The following should
work:

```python
from langchain.document_loaders import UnstructuredPowerPointLoader

filename = "../unstructured/example-docs/fake-power-point.pptx"
loader = UnstructuredPowerPointLoader(filename)
loader.load()
```

and the following should give you a `ValueError` and invite you to
upgrade `unstructured`.


```python
from langchain.document_loaders import UnstructuredPowerPointLoader

filename = "../unstructured/example-docs/fake-power-point.ppt"
loader = UnstructuredPowerPointLoader(filename)
loader.load()
```

langchain/document_loaders/powerpoint.py

@@ -1,13 +1,42 @@
 """Loader that loads powerpoint files."""
+import os
 from typing import List
 
+from unstructured.__version__ import __version__ as __unstructured_version__
+from unstructured.file_utils.filetype import FileType, detect_filetype
+
 from langchain.document_loaders.unstructured import UnstructuredFileLoader
 
+unstructured_version = tuple([int(x) for x in __unstructured_version__.split(".")])
+
 
 class UnstructuredPowerPointLoader(UnstructuredFileLoader):
     """Loader that uses unstructured to load powerpoint files."""
 
     def _get_elements(self) -> List:
-        from unstructured.partition.pptx import partition_pptx
+        # NOTE(MthwRobinson) - magic will raise an import error if the libmagic
+        # system dependency isn't installed. If it's not installed, we'll just
+        # check the file extension
+        try:
+            import magic  # noqa: F401
+
+            is_ppt = detect_filetype(self.file_path) == FileType.PPT
+        except ImportError:
+            _, extension = os.path.splitext(self.file_path)
+            is_ppt = extension == ".ppt"
+
+        if is_ppt and unstructured_version < (0, 4, 11):
+            raise ValueError(
+                f"You are on unstructured version {__unstructured_version__}. "
+                "Partitioning .ppt files is only supported in unstructured>=0.4.11. "
+                "Please upgrade the unstructured package and try again."
+            )
+
+        if is_ppt:
+            from unstructured.partition.ppt import partition_ppt
+
+            return partition_ppt(filename=self.file_path)
+        else:
+            from unstructured.partition.pptx import partition_pptx
 
-        return partition_pptx(filename=self.file_path)
+            return partition_pptx(filename=self.file_path)