replace 'tox' with 'nox'

.flake8

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+[flake8]
+max-line-length=120

CONTRIBUTING.md

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+# Contributing
+
+## Prerequisites
+
+### For Development
+
+- [Poetry](https://python-poetry.org/)
+
+### For Running Test Matrix
+
+- python 3.5
+- python 3.7
+- python 3.8
+
+*multiple python interpreters can be installed and managed using [pyenv](https://github.com/pyenv/pyenv)*
+
+## Tasks
+
+Note that a [`Makefile`](./Makefile) is provided for common tasks
+
+### Installing Dependencies
+
+    make install
+
+### Running tests
+
+    make test
+
+### Running Matrix Tests
+
+*(requires multiple Python interpreters to be installed)*
+
+    make test_matrix
+
+### Lint
+
+    make lint

Makefile

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+.DEFAULT_GOAL := all
+
+test:
+	poetry run nosetests -w tests
+
+test_matrix:
+	poetry run nox --session test
+
+lint:
+	poetry run nox --session lint
+
+all:
+	poetry run nox

noxfile.py

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+import nox
+import tempfile
+from pathlib import Path
+import os
+from distutils.dir_util import copy_tree
+
+python_versions = [
+    "3.8",
+]
+
+sphinx_versions = [
+    "^3.0",
+]
+
+@nox.session(python=python_versions, reuse_venv=True)
+@nox.parametrize("sphinx", sphinx_versions)
+def tests(session, sphinx):
+    # see https://github.com/python-poetry/poetry/issues/2920#issuecomment-693147409
+    with tempfile.TemporaryDirectory() as tmp_dir:
+        copy_tree(
+            Path.cwd(), tmp_dir
+        )
+        os.chdir(tmp_dir)
+
+        session.run('poetry', 'add', f'sphinx@{sphinx}', external=True)
+        session.run('poetry', 'install', external=True)
+        session.run('nosetests', '-w', 'tests')
+
+@nox.session(python=None)
+def lint(session):
+    session.run('flake8', 'sphinxcontrib', external=True)