DOCS: Fix Sample Code for Compatibility with Pydantic 2.0 (#13890)

- **Description:** 
I encountered an issue while running the existing sample code on the
page https://python.langchain.com/docs/modules/agents/how_to/agent_iter
in an environment with Pydantic 2.0 installed. The following error was
triggered:

```python
ValidationError                           Traceback (most recent call last)
<ipython-input-12-2ffff2c87e76> in <cell line: 43>()
     41 
     42 tools = [
---> 43     Tool(
     44         name="GetPrime",
     45         func=get_prime,

2 frames
/usr/local/lib/python3.10/dist-packages/pydantic/v1/main.py in __init__(__pydantic_self__, **data)
    339         values, fields_set, validation_error = validate_model(__pydantic_self__.__class__, data)
    340         if validation_error:
--> 341             raise validation_error
    342         try:
    343             object_setattr(__pydantic_self__, '__dict__', values)

ValidationError: 1 validation error for Tool
args_schema
  subclass of BaseModel expected (type=type_error.subclass; expected_class=BaseModel)
```

I have made modifications to the example code to ensure it functions
correctly in environments with Pydantic 2.0.

docs/docs/modules/agents/how_to/agent_iter.ipynb

@@ -22,7 +22,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "import pydantic\n",
+    "from pydantic.v1 import BaseModel, Field\n",
     "from langchain.agents import AgentType, initialize_agent\n",
     "from langchain.agents.tools import Tool\n",
     "from langchain.chains import LLMMathChain\n",
@@ -65,12 +65,12 @@
     "primes = {998: 7901, 999: 7907, 1000: 7919}\n",
     "\n",
     "\n",
-    "class CalculatorInput(pydantic.BaseModel):\n",
-    "    question: str = pydantic.Field()\n",
+    "class CalculatorInput(BaseModel):\n",
+    "    question: str = Field()\n",
     "\n",
     "\n",
-    "class PrimeInput(pydantic.BaseModel):\n",
-    "    n: int = pydantic.Field()\n",
+    "class PrimeInput(BaseModel):\n",
+    "    n: int = Field()\n",
     "\n",
     "\n",
     "def is_prime(n: int) -> bool:\n",