Fix: Migración a pyproject.toml y poetry

- Reemplaza setup.py/setup.cfg por pyproject.toml (PEP 621) + poetry.lock
- Dev dependencies en [tool.poetry.group.dev.dependencies]
- Elimina requirements_dev.txt (deps dev en pyproject.toml)
- Actualiza tox.ini, Makefile, Makefile.dev, Dockerfile y docs

tox.ini

@@ -1,27 +1,18 @@
 [tox]
-envlist = py39, py310, py311, py312, py313, flake8
+envlist = py310, py311, py312, py313, flake8
 
-[travis]
-python =
-    3.9: py39
-    3.10: py310
-    3.11: py311
-    3.12: py312
-    3.13: py313
+[flake8]
+exclude = docs, build, venv, .tox, .git, .pytest_cache, *.egg-info
 
 [testenv:flake8]
-basepython = python
-deps = flake8
-commands = flake8 facho
+basepython = python3.13
+deps = flake8>=7.0.0
+commands = flake8 facho tests examples
 
 [testenv]
 setenv =
     PYTHONPATH = {toxinidir}
-deps =
-    -r{toxinidir}/requirements_dev.txt
-; If you want to make tox run the tests with the same versions, create a
-; requirements.txt with the pinned versions and uncomment the following line:
-;     -r{toxinidir}/requirements.txt
+deps = pytest>=9.1.1,<10.0.0
 commands =
     pip install -U pip
     py.test --basetemp={envtmpdir}