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.

docs/modules/indexes/document_loaders/examples/directory_loader.ipynb

@@ -11,7 +11,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 1,
+   "execution_count": 3,
    "id": "019d8520",
    "metadata": {},
    "outputs": [],
@@ -128,10 +128,69 @@
     "len(docs)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "598a2805",
+   "metadata": {},
+   "source": [
+    "If you need to load Python source code files, use the `PythonLoader`."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "id": "c558bd73",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from langchain.document_loaders import PythonLoader"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 13,
+   "id": "a3cfaba7",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "loader = DirectoryLoader('../../../../../', glob=\"**/*.py\", loader_cls=PythonLoader)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 14,
+   "id": "e2e1e26a",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "docs = loader.load()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 15,
+   "id": "ffb8ff36",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "691"
+      ]
+     },
+     "execution_count": 15,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "len(docs)"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "984c8429",
+   "id": "7f6e0eae",
    "metadata": {},
    "outputs": [],
    "source": []
@@ -153,7 +212,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.9.1"
+   "version": "3.10.3"
   }
  },
  "nbformat": 4,