Add PythonLoader which auto-detects encoding of Python files (#3311)

This PR contributes a `PythonLoader`, which inherits from
`TextLoader` but detects and sets the encoding automatically.

tests/integration_tests/document_loaders/test_python.py

@@ -0,0 +1,19 @@
+from pathlib import Path
+
+import pytest
+
+from langchain.document_loaders.python import PythonLoader
+
+
+@pytest.mark.parametrize("filename", ["default-encoding.py", "non-utf8-encoding.py"])
+def test_python_loader(filename: str) -> None:
+    """Test Python loader."""
+    file_path = Path(__file__).parent.parent / "examples" / filename
+    loader = PythonLoader(str(file_path))
+    docs = loader.load()
+
+    assert len(docs) == 1
+
+    metadata = docs[0].metadata
+
+    assert metadata["source"] == str(file_path)