Add tox configuration

Tox manages test environments for all supported Python versions, as
well as linting and formatting tools. On GitHub, the test and lint
steps are kept as close as possible to the Makefile equivalents.

tox.ini

@@ -0,0 +1,47 @@
+[tox]
+env_list =
+    py37,py38,py39,py310,py311
+minversion = 4.5.1
+
+[gh-actions]
+# Map Github Actions Python version to environment factors
+# Requires tox-gh-actions 3.x is installed in the GitHub action
+python =
+    3.7: py37
+    3.8: py38
+    3.9: py39
+    3.10: py310
+    3.11: py311
+
+[testenv]
+description = Run the tests with pytest
+package = wheel
+extras = all
+set_env =
+    # trick poetry into adopting the tox virtualenv
+    POETRY_VIRTUALENVS_PATH = {[tox]work_dir}
+allowlist_externals = poetry
+commands_pre =
+    poetry install --no-root --sync --all-extras
+commands =
+    python -X dev -m pytest {tty:--color=yes} {posargs}
+
+[testenv:lint]
+description = Run the linters
+skip_install = true
+commands_pre =
+    poetry install --no-root --with=linting --sync --all-extras
+commands =
+    black --check --diff .
+    flake8
+    mypy
+    pyright
+
+[testenv:format]
+description = Format the code
+skip_install = true
+commands_pre =
+    poetry install --no-root --sync --with=linting
+commands =
+    black .
+    isort .