chore: fix tox.ini to install dev deps from pyproject.toml

Replace the missing requirements-dev.txt reference with extras = dev,
which installs the [dev] optional dependencies declared in pyproject.toml.
Also remove skipsdist so tox installs the package before running tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tox.ini

@@ -1,9 +1,8 @@
 [tox]
 envlist = py, lint, mypy
-skipsdist = True
 
 [testenv]
-deps = -rrequirements-dev.txt
+extras = dev
 commands =
     pytest -q