build: consolidate packaging on pyproject.toml and correct the dependency set

The project carried three sets of packaging metadata — pyproject.toml
[project], setup.cfg [options] and setup.py kwargs — declaring different
dependencies. pyproject.toml wins under any modern backend, so the wheel
was correct, but the other two said "Pillow, numpy" and reading either
gave the wrong answer about what the library needs.

Reduce setup.cfg to its [flake8] section and setup.py to a setup() shim,
each pointing at pyproject.toml.

Correct the authoritative list while consolidating:

- flask was a runtime dependency but is imported only by the fixture HTTP
  server in tests. Every user was installing Flask, Jinja2, Werkzeug,
  click, itsdangerous and blinker for nothing.
- ebooklib was a runtime dependency and is never imported by the library;
  epub_reader.py uses zipfile + xml.etree directly. Only the tests use it,
  to build EPUB fixtures.
- requests was declared required but concrete/image.py imports it lazily
  and degrades gracefully when absent, so it belongs in an extra.
- requires-python said >=3.6, which cannot be true: the package uses
  dataclasses and `from __future__ import annotations`, both 3.7+, and CI
  tests 3.10/3.12/3.13.

Runtime install drops from 7 direct dependencies to 4. Adds test,
remote-images and dev extras, and a CI step that installs into an empty
venv with only the runtime deps and imports every subpackage — this class
of defect is only caught by installing what you ship.

Verified: runtime-only install imports all subpackages; `.[test]` runs the
full suite, 853 passed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

setup.py

@@ -1,32 +1,10 @@
-from setuptools import setup, find_packages
+"""Shim for legacy `python setup.py` invocations.
 
-setup(
-    name="pyWebLayout",
-    version="0.1.1",
-    packages=find_packages(),
-    install_requires=[
-        "Pillow",
-        "numpy",
-    ],
-    extras_require={
-        "test": [
-            "coverage>=5.0",
-        ],
-        "dev": [
-            "coverage>=5.0",
-            "pytest>=6.0",
-        ],
-    },
-    author="Duncan Tourolle",
-    author_email="duncan@tourolle.paris",
-    description="A Python library for HTML-like layout and rendering",
-    long_description=open("README.md").read(),
-    long_description_content_type="text/markdown",
-    url="https://gitea.tourolle.paris/pyWebLayout",
-    classifiers=[
-        "Programming Language :: Python :: 3",
-        "License :: OSI Approved :: MIT License",
-        "Operating System :: OS Independent",
-    ],
-    python_requires=">=3.6",
-)
+All packaging metadata lives in setup.cfg. Keeping a second copy here was an
+active hazard: keyword arguments passed to setup() override setup.cfg, so the
+two could disagree silently and the setup.py copy would win.
+"""
+
+from setuptools import setup
+
+setup()