Try to zip your files using CLI directly using:

zip -r deployment_package.zip lambda_function.py boto3/ psycopg2/ requests/

this will make sure that you have your lambda_function.py in the root directory.

for the lambda_handler, try to define from the lambda aws console:

def lambda_handler(event, context):
    pass 

and see if you will keep getting the same error. you already got another structure for your files, so you may have uploaded a wrong zip file.