docs(aws-s3-deployment): update python version of a document (#22800)

----

related to #21148

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-s3-deployment/README.md

@@ -366,11 +366,11 @@ new cdk.CfnOutput(this, 'ObjectKey', {
 
 ## Development
 
-The custom resource is implemented in Python 3.7 in order to be able to leverage
+The custom resource is implemented in Python 3.9 in order to be able to leverage
 the AWS CLI for "aws s3 sync". The code is under [`lib/lambda`](https://github.com/aws/aws-cdk/tree/main/packages/%40aws-cdk/aws-s3-deployment/lib/lambda) and
 unit tests are under [`test/lambda`](https://github.com/aws/aws-cdk/tree/main/packages/%40aws-cdk/aws-s3-deployment/test/lambda).
 
-This package requires Python 3.7 during build time in order to create the custom
+This package requires Python 3.9 during build time in order to create the custom
 resource Lambda bundle and test it. It also relies on a few bash scripts, so
 might be tricky to build on Windows.