upload_report does not close opened file handles

[DreierF]

Calling upload_report leads to the following warning:

/usr/local/lib/python3.9/site-packages/teamscale_client/client.py:266: ResourceWarning: unclosed file <_io.BufferedReader name='testwise-coverage.json'>
  return self.upload_report(coverage_files, coverage_format, timestamp, message, partition)
ResourceWarning: Enable tracemalloc to get the object allocation traceback

The file handles should be properly closed after the upload.