Stringify `AgentType` before saving to yaml (#2998)
Code to reproduce the issue (with `langchain==0.0.141`):
```python
from langchain.agents import initialize_agent, load_tools
from langchain.llms import OpenAI
llm = OpenAI(temperature=0.9, verbose=True)
tools = load_tools(["llm-math"], llm=llm)
agent = initialize_agent(tools, llm, agent="zero-shot-react-description", verbose=True)
agent.save_agent("agent.yaml")
with open("agent.yaml") as f:
print(f.read())
```
Output:
```
_type: !!python/object/apply:langchain.agents.agent_types.AgentType
- zero-shot-react-description
allowed_tools:
- Calculator
...
```
I expected `_type` to be `zero-shot-react-description` but it's actually
not. This PR fixes it by stringifying `AgentType` (`Enum`).
Signed-off-by: harupy <hkawamura0130@gmail.com>
