checkbox-cli crashes with 'I/O operation on closed file'

Run checkbox-cli, select smoke tests and run to completion.

The app crashes with:

Traceback (most recent call last):
  File "/home/zyga/checkbox/plainbox/plainbox/impl/clitools.py", line 513, in dispatch_and_catch_exceptions
    return self.dispatch_command(ns)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/clitools.py", line 509, in dispatch_command
    return ns.command.invoked(ns)
  File "/home/zyga/checkbox/checkbox-ng/checkbox_ng/commands/cli.py", line 609, in invoked
    self.settings, ns).run()
  File "/home/zyga/checkbox/checkbox-ng/checkbox_ng/commands/cli.py", line 364, in run
    self._run_jobs(runner, ns, manager)
  File "/home/zyga/checkbox/checkbox-ng/checkbox_ng/commands/cli.py", line 413, in _run_jobs
    self.save_results(manager)
  File "/home/zyga/checkbox/checkbox-ng/checkbox_ng/commands/cli.py", line 459, in save_results
    exporter.dump(data_subset, stream)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/exporter/xlsx.py", line 583, in dump
    self.write_summary(data)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/exporter/xlsx.py", line 319, in write_summary
    self.total_fail, self.total)
ValueError: Single '}' encountered in format string
Traceback (most recent call last):
  File "/ramdisk/venv/bin/checkbox-cli", line 9, in <module>
    load_entry_point('checkbox-ng==0.4.dev', 'console_scripts', 'checkbox-cli')()
  File "/home/zyga/checkbox/checkbox-ng/checkbox_ng/main.py", line 174, in checkbox_cli
    CheckBoxNGTool().main(['checkbox-cli'] + args))
  File "/home/zyga/checkbox/plainbox/plainbox/impl/clitools.py", line 291, in main
    return self.dispatch_and_catch_exceptions(ns)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/clitools.py", line 513, in dispatch_and_catch_exceptions
    return self.dispatch_command(ns)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/clitools.py", line 509, in dispatch_command
    return ns.command.invoked(ns)
  File "/home/zyga/checkbox/checkbox-ng/checkbox_ng/commands/cli.py", line 609, in invoked
    self.settings, ns).run()
  File "/home/zyga/checkbox/checkbox-ng/checkbox_ng/commands/cli.py", line 364, in run
    self._run_jobs(runner, ns, manager)
  File "/home/zyga/checkbox/checkbox-ng/checkbox_ng/commands/cli.py", line 413, in _run_jobs
    self.save_results(manager)
  File "/home/zyga/checkbox/checkbox-ng/checkbox_ng/commands/cli.py", line 459, in save_results
    exporter.dump(data_subset, stream)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/exporter/xlsx.py", line 583, in dump
    self.write_summary(data)
  File "/home/zyga/checkbox/plainbox/plainbox/impl/exporter/xlsx.py", line 319, in write_summary
    self.total_fail, self.total)
ValueError: Single '}' encountered in format string
Exception ignored in: <bound method Workbook.__del__ of <xlsxwriter.workbook.Workbook object at 0x7fc4b75beda0>>
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/xlsxwriter/workbook.py", line 127, in __del__
    self.close()
  File "/usr/lib/python3/dist-packages/xlsxwriter/workbook.py", line 237, in close
    self._store_workbook()
  File "/usr/lib/python3/dist-packages/xlsxwriter/workbook.py", line 420, in _store_workbook
    xlsx_file.write(os_filename, xml_filename)
  File "/usr/lib/python3.4/zipfile.py", line 1343, in write
    zinfo.header_offset = self.fp.tell() # Start of header bytes
ValueError: I/O operation on closed file
Exception ignored in: <bound method ZipFile.__del__ of <zipfile.ZipFile object at 0x7fc4b40d0828>>
Traceback (most recent call last):
  File "/usr/lib/python3.4/zipfile.py", line 1457, in __del__
    self.close()
  File "/usr/lib/python3.4/zipfile.py", line 1468, in close
    pos1 = self.fp.tell()
ValueError: I/O operation on closed file