Add tox to pyproject

pyproject.toml

@@ -11,4 +11,28 @@ addopts = """
     --cov=nexgen --cov-report=xml --cov-branch
     """
 junit_family = "xunit2"
-testpaths = "src tests"
+testpaths = "src tests"
+
+# tox must currently be configured via an embedded ini string
+# See: https://github.com/tox-dev/tox/issues/999
+[tool.tox]
+legacy_tox_ini = """
+[tox]
+skipsdist=True
+
+[testenv:{pre-commit,mypy,pytest,docs}]
+# Don't create a virtualenv for the command, requires tox-direct plugin
+direct = True
+passenv = *
+allowlist_externals =
+    pytest
+    pre-commit
+    mypy
+    sphinx-build
+    sphinx-autobuild
+commands =
+    pytest: pytest {posargs}
+    mypy: mypy src tests --ignore-missing-imports --no-strict-optional {posargs}
+    pre-commit: pre-commit run --all-files {posargs}
+    docs: sphinx-{posargs:build -EW --keep-going} -T docs build/html
+"""