Fix CI by dropping EOL Python versions

This commit introduces the following major changes:

* Use tox.
* Test the project against both `unidecode` and `text_unidecode`.
  Previously, `unidecode` was untested.
* Drop support for Python 3.7, 3.8, and 3.9.
  CI was unable to run because Python 3.7 is no longer available.
  In addition, Python 3.9 and lower are all end-of-life.
* Test against PyPy 3.11 and Python 3.14.
* Test type annotations using mypy.
  Previously, type annotations were untested.
* Install and test all supported Python interpreters in CI at once.
* Use tox, and the tox-uv plugin, exclusively in CI.
  This eliminates differences between local and CI testing.
* Run CI against pull requests.
* Collect, combine, and report coverage
  for all Python versions and dependencies, locally and in CI.

In addition, this commit introduces the following minor changes:

* Fix typing issues identified by mypy.
  Note that `text_unidecode` is untyped and must be type-ignored.
* Remove almost all coverage `pragma: nocover` lines.
* Fix incorrect copy/pasted comments in `ci.yml` and `main.yml`.
* Upgrade `actions/checkout` to v5 and `actions/setup-python` to v6.

Reintroduce tox and thoroughly test the project

`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.
```

The `tests_require` option throws a `UserWarning`:

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

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.
        ********************************************************************************

!!
```

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 pypa/pip#11457
```

Fix build process

Fix the README badge to refer to the `main.yml` workflow results