<!--- 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

```

"""
    :type args: CoverageConfig
    """
    output_files = command_coverage_combine(args)
    for output_file in output_files:
        if output_file.endswith('-powershell'):
            
            display.info("Skipping output file %s in html generation" % output_file, verbosity=3)
            continue
        dir_name = os.path.join(ResultType.REPORTS.path, os.path.basename(output_file))
        make_dirs(dir_name)
        run_coverage(args, output_file, 'html', ['-i', '-d', dir_name])
        display.info('HTML report generated: file:///%s' % os.path.join(dir_name, 'index.html'))
def command_coverage_xml(args):