Update the build-system requirements; resolve warnings and errors (#327)

* Update the build-system requirements; resolve warnings and errors

# The current situation

The current build-system requires setuptools 40.8.0 or higher.
However, setuptools v40.8.0 doesn't have a `build_meta` submodule.

To demonstrate the problem, the following commands were run:

```
python3.13 -m venv venv-3.13
source venv-3.13/bin/activate
python -m pip install setuptools==40.8.0 build
python -m build --no-isolation
```

This results in the following error message:

```
$ python -m build --no-isolation
* Getting build dependencies for sdist...

Traceback (most recent call last):
  File ".../lib/python3.13/site-packages/pyproject_hooks/_impl.py", line 402, in _call_hook
    raise BackendUnavailable(
    ...<4 lines>...
    )
pyproject_hooks._impl.BackendUnavailable: Cannot import 'setuptools.build_meta'

ERROR Backend 'setuptools.build_meta' is not available.
```

Upgrading to the latest version of setuptools, 80.9.0,
allows the package to build:

```
python -m pip install --upgrade setuptools
python -m build --no-isolation
```

However, warnings are emitted:

```
SetuptoolsDeprecationWarning: `project.license` as a TOML table is deprecated
!!

********************************************************************************
Please use a simple string containing a SPDX expression for `project.license`.
You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

By 2026-Feb-18, you need to update your project and remove deprecated calls
or your builds will no longer be supported.

See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
********************************************************************************
```

```
SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

********************************************************************************
Please consider removing the following classifiers in favor of a SPDX license expression:

License :: OSI Approved :: Apache Software License

See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
********************************************************************************
```

# The solution

This change addresses the lower setuptools bound based on the warnings:
it sets the setuptools minimum version to v77.0.1
(v77.0.0 was not released to PyPI).

In addition, it addresses the license-related warnings
by switching to the SPDX short identifier `"Apache-2.0"`
and by removing the `License` trove classifier.

With this change in place, the package can be built
on the lowest supported setuptools version
as well as the latest released version.

* Feedback: Require `setuptools>=77.0.3`

From the PR comment:

> 77.0.3 fixes known early bugs of the PEP 639 implementation

---------

Co-authored-by: Mike Fiedler <miketheman@gmail.com>

Author: kurtmckee

pyproject.toml

@@ -6,15 +6,14 @@ authors = [
   {name = "The Python Packaging Authority", email = "admin@mail.pypi.org"}
 ]
 readme = "README.rst"
-license = {text = "Apache License, Version 2.0"}
+license = "Apache-2.0"
 dependencies = [
   "nh3>=0.2.14",
   "docutils>=0.21.2",
   "Pygments>=2.5.1",
 ]
 classifiers = [
   "Intended Audience :: Developers",
-  "License :: OSI Approved :: Apache Software License",
   "Natural Language :: English",
   "Operating System :: MacOS :: MacOS X",
   "Operating System :: POSIX",
@@ -48,7 +47,7 @@ exclude = ["tests", "tests.*"]
 Home-page = "https://github.com/pypa/readme_renderer"
 
 [build-system]
-requires = ["setuptools>=40.8.0"]
+requires = ["setuptools>=77.0.3"]
 build-backend = "setuptools.build_meta"