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

CONTRIBUTING.rst

@@ -69,7 +69,7 @@ Ready to contribute? Here's how to set up `facho` for local development.
     $ python3 -mvenv facho-venv && source facho-venv/bin/activate
     $ cd facho/
     $ pre-commit install
-    $ python setup.py develop
+    $ pip install -e .
 
 4. Create a branch for local development::
 
@@ -81,7 +81,7 @@ Ready to contribute? Here's how to set up `facho` for local development.
    tests, including testing other Python versions with tox::
 
     $ flake8 facho tests
-    $ python setup.py test or py.test
+    $ py.test
     $ tox
 
    To get flake8 and tox, just pip install them into your virtualenv.
@@ -97,10 +97,10 @@ Ready to contribute? Here's how to set up `facho` for local development.
 Using docker
 ------------
 
-1. make -f Makefile.dev build
+1. make -f Makefile.dev dev-setup
 2. make -f Makefile.dev dev-shell
-3. make -f Makefile.dev python3.8 setup.py develop
-4. make -f Makefile.dev python3.8 setup.py test
+3. make -f Makefile.dev test
+4. make -f Makefile.dev tox
 
 Pull Request Guidelines
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