<!--- Verify first that your issue is not already reported on GitHub -->
<!--- Also test if the latest release and devel branch are affected too -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### SUMMARY
<!--- Explain the problem briefly below -->
traceback with --all option with ansible-test coverage
##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
/bin/ansible-test

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
stable 2.9
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below

```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->


##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->
Run coverage with below comand
<!--- Paste example playbooks or commands between quotes below -->
```yaml
ansible-test coverage html --all 
```

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->
No traceback should occure

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->

<!--- Paste verbatim command output between quotes -->
```paste below
Traceback (most recent call last):
  File "/home/abehl/work/src/anshul_ansible/ansible/bin/ansible-test", line 28, in <module>
    main()
  File "/home/abehl/work/src/anshul_ansible/ansible/bin/ansible-test", line 24, in main
    cli_main()
  File "/home/abehl/work/src/anshul_ansible/ansible/test/lib/ansible_test/_internal/cli.py", line 128, in main
    args.func(config)
  File "/home/abehl/work/src/anshul_ansible/ansible/test/lib/ansible_test/_internal/cover.py", line 343, in command_coverage_html
    output_files = command_coverage_combine(args)
  File "/home/abehl/work/src/anshul_ansible/ansible/test/lib/ansible_test/_internal/cover.py", line 73, in command_coverage_combine
    paths = _command_coverage_combine_powershell(args) + _command_coverage_combine_python(args)
  File "/home/abehl/work/src/anshul_ansible/ansible/test/lib/ansible_test/_internal/cover.py", line 182, in _command_coverage_combine_python
    updated.write_file(output_file)
  File "/home/abehl/work/src/anshul_ansible/ansible/venv3/lib/python3.7/site-packages/coverage/data.py", line 468, in write_file
    self.write_fileobj(fdata)
  File "/home/abehl/work/src/anshul_ansible/ansible/venv3/lib/python3.7/site-packages/coverage/data.py", line 461, in write_fileobj
    json.dump(file_data, file_obj, separators=(',', ':'))
  File "/usr/lib64/python3.7/json/__init__.py", line 179, in dump
    for chunk in iterable:
  File "/usr/lib64/python3.7/json/encoder.py", line 431, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/usr/lib64/python3.7/json/encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "/usr/lib64/python3.7/json/encoder.py", line 376, in _iterencode_dict
    raise TypeError(f'keys must be str, int, float, bool or None, '
TypeError: keys must be str, int, float, bool or None, not tuple

```

else:
                missing.append('%s-%s' % (current_missing, end_line))
        file_report.append({'name': filename, 'stmts': stmts, 'miss': miss, 'cover': cover, 'missing': missing})
    if total_stmts == 0:
        return ''
    total_percent = '{0}%'.format(int((total_stmts - total_miss) / total_stmts * 100))
    stmts_padding = max(8, len(str(total_stmts)))
    miss_padding = max(7, len(str(total_miss)))
    line_length = name_padding + stmts_padding + miss_padding + cover_padding
    header = 'Name'.ljust(name_padding) + 'Stmts'.rjust(stmts_padding) + 'Miss'.rjust(miss_padding) + \
             'Cover'.rjust(cover_padding)
    if args.show_missing:
        header += 'Lines Missing'.rjust(16)
        line_length += 16
    line_break = '-' * line_length
    lines = ['%s%s%s%s%s' % (f['name'].ljust(name_padding), str(f['stmts']).rjust(stmts_padding),
                             str(f['miss']).rjust(miss_padding), f['cover'].rjust(cover_padding),
                             '   ' + ', '.join(f['missing']) if args.show_missing else '')
             for f in file_report]
    totals = 'TOTAL'.ljust(name_padding) + str(total_stmts).rjust(stmts_padding) + \
             str(total_miss).rjust(miss_padding) + total_percent.rjust(cover_padding)
    report = '{0}\n{1}\n{2}\n{1}\n{3}'.format(header, line_break, "\n".join(lines), totals)
    return report
def run_coverage(args, output_file, command, cmd):  
    """Run the coverage cli tool with the specified options."""
    env = common_environment()
    env.update(dict(COVERAGE_FILE=output_file))
    cmd = ['python', '-m', 'coverage', command, '--rcfile', COVERAGE_CONFIG_PATH] + cmd
    intercept_command(args, target_name='coverage', env=env, cmd=cmd, disable_coverage=True)