[fix] Fixed development status in setup.py

[pandafy]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Description of Changes

Without this fix, the package will not upload to PyPI.

twine upload dist/* gives the following error

ERROR    HTTPError: 400 Bad Request from https://upload.pypi.org/legacy/                                                        
         'Development Status :: 5 - Production/Stable ' is not a valid classifier. See                                          
         https://packaging.python.org/specifications/core-metadata for more information. 

Moreover, the PyPI CI task was not triggered for 1.1.1 release.

[coveralls]

coverage: 99.2%. remained the same
when pulling 258e690 on fix-setup.py
into 4690e56 on master.