aws_cdk.aws_lambda.Function: Python's `Function` uses Node.js' `__dirname`

### Describe the issue

### Example
```py
import aws_cdk.aws_signer as signer

signing_profile = signer.SigningProfile(self, "SigningProfile",
platform=signer.Platform.AWS_LAMBDA_SHA384_ECDSA
)

code_signing_config = lambda_.CodeSigningConfig(self, "CodeSigningConfig",
signing_profiles=[signing_profile]
)

lambda_.Function(self, "Function",
code_signing_config=code_signing_config,
runtime=lambda_.Runtime.NODEJS_18_X,
handler="index.handler",
code=lambda_.Code.from_asset(path.join(__dirname, "lambda-handler"))
)
```

The JSII's autogenerated code uses Node.js' `__dirname`, which isn't valid in Python.

Ideally, you should want to use [pathlib](https://docs.python.org/3/library/pathlib.html) as best practice:
```py
code=lambda_.Code.from_asset(
str(Path(__file__).parent / "lambda-handler")
)
```

Also, the default Lambda function's handler in Python is:
```py
handler="lambda_function.lambda_handler"
```

### Links

https://docs.aws.amazon.com/cdk/api/v2/python/aws_cdk.aws_lambda/Function.html

Start with the Python Function API documentation linked in the issue and trace how the JSII-generated Python example is produced. Check how the asset path and default Lambda handler are represented for Python, then verify that the generated example uses pathlib-compatible syntax and the Python handler value shown in the issue.

Written by the indexing model from the issue text.