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>

.gitea/workflows/ci.yml

@@ -24,17 +24,29 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
           
+      - name: Verify declared dependencies are sufficient
+        run: |
+          # A clean venv with ONLY the declared runtime deps. If an import here
+          # fails, install_requires is incomplete and a real `pip install
+          # pyWebLayout` would fail the same way for a user.
+          python -m venv /tmp/clean-install
+          /tmp/clean-install/bin/pip install --upgrade pip
+          /tmp/clean-install/bin/pip install .
+          /tmp/clean-install/bin/python -c "
+          import pyWebLayout.concrete, pyWebLayout.abstract
+          import pyWebLayout.io.readers.epub_reader
+          import pyWebLayout.io.readers.html_extraction
+          import pyWebLayout.layout.ereader_manager
+          print('clean install imports OK')
+          "
+
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          # Install package in development mode
-          pip install -e .
-          # Install test dependencies if they exist
-          if [ -f requirements-dev.txt ]; then pip install -r requirements-dev.txt; fi
-          if [ -f requirements/test.txt ]; then pip install -r requirements/test.txt; fi
-          # Install common test packages
-          pip install pytest pytest-cov flake8 coverage-badge interrogate
-          
+          # Install package in development mode, with the declared dev extra.
+          # Test dependencies belong in setup.cfg, not in an ad-hoc pip line.
+          pip install -e '.[dev]'
+
       - name: Download initial failed badges
         run: |
           echo "Downloading initial failed badges..."