Update Python CLI template imports to use underscore notation (#417)

Python packages with dots in their names must be imported using
underscores. The CLI templates were using the incorrect dot notation
(`microsoft.teams.*`) instead of the required underscore notation
(`microsoft_teams.*`).

## Changes

Updated import statements in all three Python CLI templates:

**Before:**
```python
from microsoft.teams.api import MessageActivity
from microsoft.teams.apps import App
from microsoft.teams.devtools import DevToolsPlugin
```

**After:**
```python
from microsoft_teams.api import MessageActivity
from microsoft_teams.apps import App
from microsoft_teams.devtools import DevToolsPlugin
```

## Files Modified

- `packages/cli/templates/python/ai/src/main.py` - Updated 4 imports
(ai, apps, devtools, openai)
- `packages/cli/templates/python/echo/src/main.py` - Updated 3 imports
(api, apps, devtools)
- `packages/cli/templates/python/graph/src/main.py` - Updated 2 imports
(api, apps)

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<details>

<summary>Original prompt</summary>

> For python cli templates, all imports must change from
microsoft.teams.* to microsoft_teams.*. So for eg. microsoft.teams.apps
changes to microsoft_teams.apps. Can you make those changes please?


</details>



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packages/cli/templates/python/ai/src/main.py

@@ -2,10 +2,10 @@
 import os
 
 from dotenv import load_dotenv
-from microsoft.teams.ai import ChatPrompt
-from microsoft.teams.apps import App
-from microsoft.teams.devtools import DevToolsPlugin
-from microsoft.teams.openai import OpenAICompletionsAIModel
+from microsoft_teams.ai import ChatPrompt
+from microsoft_teams.apps import App
+from microsoft_teams.devtools import DevToolsPlugin
+from microsoft_teams.openai import OpenAICompletionsAIModel
 
 # Load variables from a .env file (if present) so os.environ.get() will pick them up.
 # Developers should copy `sample.env` -> `.env` to provide local values.

packages/cli/templates/python/echo/src/main.py

@@ -1,9 +1,9 @@
 import asyncio
 import re
 
-from microsoft.teams.api import MessageActivity, TypingActivityInput
-from microsoft.teams.apps import ActivityContext, App
-from microsoft.teams.devtools import DevToolsPlugin
+from microsoft_teams.api import MessageActivity, TypingActivityInput
+from microsoft_teams.apps import ActivityContext, App
+from microsoft_teams.devtools import DevToolsPlugin
 
 app = App(plugins=[DevToolsPlugin()])
 

packages/cli/templates/python/graph/src/main.py

@@ -7,8 +7,8 @@
 import logging
 import os
 
-from microsoft.teams.api import MessageActivity
-from microsoft.teams.apps import ActivityContext, App, SignInEvent
+from microsoft_teams.api import MessageActivity
+from microsoft_teams.apps import ActivityContext, App, SignInEvent
 
 logger = logging.getLogger(__name__)