We don't really need mypy on 3.9 (unit tests would catch any real
issues), and it causes issues with the rest of this stack.


---
[//]: # (BEGIN SAPLING FOOTER)
* openai#324
* openai#322
* openai#321
* openai#320
* __->__ openai#319

### Summary:
1. Add the MCP dep for python 3.10, since it doesn't support 3.9 and
below
2. Create MCPServer, which is the agents SDK representation of an MCP
server
3. Create implementations for HTTP-SSE and StdIO servers, directly
copying the [MCP SDK
example](https://github.com/modelcontextprotocol/python-sdk/blob/main/examples/clients/simple-chatbot/mcp_simple_chatbot/main.py)
4. Add a util to transform MCP tools into Agent SDK tools

Note: I added optional caching support to the servers. That way, if you
happen to know a server's tools don't change, you can just cache them.

### Test Plan:

Checks pass. I added tests at the end of the stack.

--- 

openai#324
openai#322
openai#321
-> openai#320
openai#319

### Summary:
This enables users to **use** MCP inside the SDK.
1. You add a list of MCP servers to `Agent`, via `mcp_server=[...]`
2. When an agent runs, we look up its MCP tools and add them to the list
of tools.
3. When a tool call occurs, we call the relevant MCP server.

Notes:
1. There's some refactoring to make sure we send the full list of tools
to the Runner/Model etc.
2. Right now, you could have a locally defined tool that conflicts with
an MCP defined tool. I didn't add errors for that, will do in a
followup.

### Test Plan:
See unit tests. Also has an end to end example next PR.

---
[//]: # (BEGIN SAPLING FOOTER)
* openai#324
* openai#322
* __->__ openai#321
* openai#320