Support slash in file path in reduce command

[fabienheureux]

Reduce commands fails when using a nested xml file path

I am using a docker setup locally, and have my xml file located in /app/dump.xml in the docker container.
I am using the reduce command as : python manage.py reduce_xml /app/dump.xml This causes the command to fail when writing the stats file as the command tries to write f = open(f"stats-{file_path}.json", "w") resulting in 'stats-/app/dump.xml.json'

Here is the full stacktrace

```python Traceback (most recent call last): File "manage.py", line 10, in execute_from_command_line(sys.argv) File "/usr/local/lib/python3.8/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line utility.execute() File "/usr/local/lib/python3.8/site-packages/django/core/management/__init__.py", line 395, in execute self.fetch_command(subcommand).run_from_argv(self.argv) File "/usr/local/lib/python3.8/site-packages/django/core/management/base.py", line 330, in run_from_argv self.execute(*args, **cmd_options) File "/usr/local/lib/python3.8/site-packages/django/core/management/base.py", line 371, in execute output = self.handle(*args, **options) File "/usr/local/lib/python3.8/site-packages/wagtail_wordpress_import/management/commands/reduce_xml.py", line 109, in handle f = open(f"stats-{file_path}.json", "w") FileNotFoundError: [Errno 2] No such file or directory: 'stats-/app/dump.xml.json' ```

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• Testing
  • [x] CI passes
  • [x] If necessary, tests are added for new or fixed behaviour
  • [x] These changes do not reduce test coverage
• Documentation.
  • [ ] This PR adds or updates documentation
  • [x] Documentation changes are not necessary because they do not change the reduce command behavior/arguments

[codecov-commenter]

Codecov Report

Merging #129 (a61b072) into main (3c27330) will increase coverage by 0.19%. The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
+ Coverage   65.38%   65.57%   +0.18%     
==========================================
  Files          32       32              
  Lines        1251     1255       +4     
  Branches      222      222              
==========================================
+ Hits          818      823       +5     
+ Misses        392      391       -1     
  Partials       41       41              

Impacted Files	Coverage Δ
...wordpress_import/management/commands/reduce_xml.py	32.30% <83.33%> (+6.07%)	:arrow_up:

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[nickmoreton]

@fabienheureux Thank you for the PR. The tests are now passing :)