title: Complete a multipart_upload with boto3?
body: <p>Tried this: </p>
  
  <pre><code>import boto3
  from boto3.s3.transfer import TransferConfig, S3Transfer
  path = "/temp/"
  fileName = "bigFile.gz" # this happens to be a 5.9 Gig file
  client = boto3.client('s3', region)
  config = TransferConfig(
      multipart_threshold=4*1024, # number of bytes
      max_concurrency=10,
      num_download_attempts=10,
  )
  transfer = S3Transfer(client, config)
  transfer.upload_file(path+fileName, 'bucket', 'key')
  </code></pre>
  
  <p>Result: 5.9 gig file on s3. Doesn't seem to contain multiple parts.</p>
  
  <p>I found <a href="https://github.com/boto/boto3/issues/50" rel="noreferrer">this example</a>, but <code>part</code> is not defined. </p>
  
  <pre><code>import boto3
  
  bucket = 'bucket'
  path = "/temp/"
  fileName = "bigFile.gz"
  key = 'key'
  
  s3 = boto3.client('s3')
  
  # Initiate the multipart upload and send the part(s)
  mpu = s3.create_multipart_upload(Bucket=bucket, Key=key)
  with open(path+fileName,'rb') as data:
      part1 = s3.upload_part(Bucket=bucket
                             , Key=key
                             , PartNumber=1
                             , UploadId=mpu['UploadId']
                             , Body=data)
  
  # Next, we need to gather information about each part to complete
  # the upload. Needed are the part number and ETag.
  part_info = {
      'Parts': [
          {
              'PartNumber': 1,
              'ETag': part['ETag']
          }
      ]
  }
  
  # Now the upload works!
  s3.complete_multipart_upload(Bucket=bucket
                               , Key=key
                               , UploadId=mpu['UploadId']
                               , MultipartUpload=part_info)
  </code></pre>
  
  <p><strong>Question: Does anyone know how to use the multipart upload with boto3?</strong></p>