Use custom css to fix external depenceny upload restriction

[nezhar]

This is required due to a restirction of PEP 440 and PEP 508 - pypi/warehouse#7136 (comment)

Log from https://github.com/anexia/django-future-tasks/actions/runs/7032213426/job/19135487570#step:7:20

Run twine upload dist/*
  twine upload dist/*
  shell: /usr/bin/bash -e {0}
  env:
    pythonLocation: /opt/hostedtoolcache/Python/3.8.18/x64
    LD_LIBRARY_PATH: /opt/hostedtoolcache/Python/3.8.18/x64/lib
    TWINE_REPOSITORY: ***
    TWINE_USERNAME: ***
    TWINE_PASSWORD: ***
    TWINE_NON_INTERACTIVE: yes
Uploading distributions to https://upload.***.org/legacy/
Uploading django_future_tasks-1.1.0-py3-none-any.whl
[2](https://github.com/anexia/django-future-tasks/actions/runs/7032213426/job/19135487570#step:7:2)5l
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25hWARNING  Error during upload. Retry with the --verbose option for more details. 
ERROR    HTTPError: [4](https://github.com/anexia/django-future-tasks/actions/runs/7032213426/job/19135487570#step:7:4)00 Bad Request from [https://upload.***.org/legacy/](https://upload.%2A%2A%2A.org/legacy/)        
         Invalid value for requires_dist. Error: Can't have direct dependency:  
         'django-cronfield @                                                    
         git+https://github.com/galipnik/[email protected]
         '                                                                      
Error: Process completed with exit code 1.

It uses the latest release of django-cronfield and moves the css fixes as owerides in this repository.

We can revert this once knaperek/django-cronfield#3 is merged and there is a new release.

[galipnik]

Changes LGTM, thanks!