可以使用以下的bash脚本来解决问题：

image: python:3.6.4-slim-stretch

pipelines:
  branches:
    master:
     - step:
        name: Build and Deploy to S3
        script:
          - apt-get update && apt-get install -y git
          - git init
          - git remote add origin "YOUR GIT URL"
          # Remove unwanted files
          - git filter-branch --force --index-filter 'git rm --cached -qr --ignore-unmatch unwanted_folder' --prune-empty --tag-name-filter cat -- --all
          # Install dependencies and build your app
          - pip install -r requirements.txt
          - python build.py
          # Configure AWS CLI
          - apt-get install -y unzip
          - curl "https://awscli.amazonaws.com/awscli-exe-linux-x86_64.zip" -o "awscliv2.zip"
          - unzip awscliv2.zip
          - ./aws/install
          - aws configure set aws_access_key_id $AWS_ACCESS_KEY_ID
          - aws configure set aws_secret_access_key $AWS_SECRET_ACCESS_KEY
          - aws configure set default.region $AWS_DEFAULT_REGION
          # Upload to S3
          - aws s3 sync build/ s3://yourbucketname --delete

在脚本中，'unwanted_folder”替换为你需要删除的文件夹名称，上传的S3桶名字替换为你自己的桶名字。此脚本删除了不需要的文件后，上传其他需要文件到S3桶中。