Fix build process

[kurtmckee]

Hello! Thanks for your work on python-slugify!

slugify's packaging is using a number of deprecated constructs, and building and installing the package throws a number of deprecation warnings. This PR resolves all of the warnings that I was seeing.

• Remove deprecated codecs.open() usage

  codecs.open() is deprecated and throws a DeprecationWarning:

  <string>:20: DeprecationWarning: codecs.open() is deprecated. Use open() instead.
  <string>:23: DeprecationWarning: codecs.open() is deprecated. Use open() instead.
  

• Remove the unknown tests_require option

  The tests_require option throws a UserWarning:

  /.../site-packages/setuptools/_distutils/dist.py:289:
  UserWarning: Unknown distribution option: 'tests_require'
    warnings.warn(msg)
  

• Resolve deprecated License trove classifier usage

  This resolves build warnings thrown by setuptools:

  /.../site-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
  !!
  
          ********************************************************************************
          Please consider removing the following classifiers in favor of a SPDX license expression:
  
          License :: OSI Approved :: MIT License
  
          See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
          ********************************************************************************
  
  !!
  

• Resolve deprecation warnings thrown when installing in editable mode

  Running pip install -e. displays the following deprecation warnings:

  DEPRECATION: Legacy editable install of python-slugify==8.0.4
  from file:///.../pr-python-slugify (setup.py develop) is deprecated.
  pip 25.3 will enforce this behaviour change.
  A possible replacement is to add a pyproject.toml
  or enable --use-pep517, and use setuptools >= 64.
  If the resulting installation is not behaving as expected,
  try using --config-settings editable_mode=compat.
  Please consult the setuptools documentation for more information.
  Discussion can be found at https://github.com/pypa/pip/issues/11457
  

---

With these changes, it's now possible to build and install the package without any warnings. Example:

# Building the package no longer throws warnings:
python -m pip install build
python -m build

# Installing in editable mode no longer throws warnings:
python -m pip install -e.

Warning

Running commands like python setup.py bdist_wheel is deprecated and may start failing at the end of this month. I strongly recommend migrating off of an executable setup.py file entirely, in favor of a declarative pyproject.toml file.

$ python setup.py bdist_wheel
...
/.../lib/python3.13/site-packages/setuptools/_distutils/cmd.py:90: SetuptoolsDeprecationWarning: setup.py install is deprecated.
!!

        ********************************************************************************
        Please avoid running ``setup.py`` directly.
        Instead, use pypa/build, pypa/installer or other
        standards-based tools.

        By 2025-Oct-31, you need to update your project and remove deprecated calls
        or your builds will no longer be supported.

        See https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html for details.
        ********************************************************************************

!!