diff --git a/Makefile b/Makefile
index f115c60..8c7c842 100644
--- a/Makefile
+++ b/Makefile
@@ -38,7 +38,7 @@ source: ## Build Python source distribution package
 	poetry build --format sdist
 
 test: clean-tox ## Run the project testsuite(s)
-	poetry run tox --parallelize-locked-install=10
+	poetry run tox
 
 publish: clean test wheel source ## Build and upload to pypi (requires $PYPI_API_KEY be set)
 	@poetry publish --username __token__ --password $(PYPI_API_KEY)
diff --git a/poetry.lock b/poetry.lock
index d33b779..d286a40 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -1351,7 +1351,7 @@ testing = ["flaky (>=3.4.0)", "freezegun (>=0.3.11)", "psutil (>=5.6.1)", "pytes
 
 [[package]]
 name = "tox-poetry-installer"
-version = "0.7.0"
+version = "0.8.0"
 description = "A plugin for Tox that lets you install test environment dependencies from the Poetry lockfile"
 category = "dev"
 optional = false
@@ -1410,7 +1410,7 @@ brotli = ["brotlipy (>=0.6.0)"]
 
 [[package]]
 name = "virtualenv"
-version = "20.4.5"
+version = "20.4.6"
 description = "Virtual Python Environment builder"
 category = "dev"
 optional = false
@@ -1478,7 +1478,7 @@ testing = ["pytest (>=4.6)", "pytest-checkdocs (>=1.2.3)", "pytest-flake8", "pyt
 [metadata]
 lock-version = "1.1"
 python-versions = "^3.7"
-content-hash = "398575a87d787fb87b2993863207aa6375eadb5c4012fc6f849e51c40b01548f"
+content-hash = "1b8a69bd62b3c5bd96d2512ad70438c3aa19a5064ca8303d45aa968432197500"
 
 [metadata.files]
 alabaster = [
@@ -2239,8 +2239,8 @@ tox = [
     {file = "tox-3.23.1.tar.gz", hash = "sha256:307a81ddb82bd463971a273f33e9533a24ed22185f27db8ce3386bff27d324e3"},
 ]
 tox-poetry-installer = [
-    {file = "tox-poetry-installer-0.7.0.tar.gz", hash = "sha256:409560bbaa6910475f224c2d1541b8c8d6215209bda395cd28c9307e8cbb861d"},
-    {file = "tox_poetry_installer-0.7.0-py3-none-any.whl", hash = "sha256:f4f158bc40c003abbb2595344d1c854da8cb36e83d3510a47db1524512bfa898"},
+    {file = "tox-poetry-installer-0.8.0.tar.gz", hash = "sha256:6df71f3ea34300b45d0642591946a0f413ce14de2c67d1b5f3c0b84ee28466be"},
+    {file = "tox_poetry_installer-0.8.0-py3-none-any.whl", hash = "sha256:b9b1fcfffb966ddc5cf78d4bdee7eca307426bbdb26b4ad534c9b880b25136c9"},
 ]
 traitlets = [
     {file = "traitlets-5.0.5-py3-none-any.whl", hash = "sha256:69ff3f9d5351f31a7ad80443c2674b7099df13cc41fc5fa6e2f6d3b0330b0426"},
@@ -2288,8 +2288,8 @@ urllib3 = [
     {file = "urllib3-1.26.4.tar.gz", hash = "sha256:e7b021f7241115872f92f43c6508082facffbd1c048e3c6e2bb9c2a157e28937"},
 ]
 virtualenv = [
-    {file = "virtualenv-20.4.5-py2.py3-none-any.whl", hash = "sha256:73b4186ee7e08ffd1a96cd5d60e5a946132f8ce4736e8f209eda3a6c35b2ceb8"},
-    {file = "virtualenv-20.4.5.tar.gz", hash = "sha256:e82dbb66e0d6ecf626f037df256f185571da552007bfa9f3f317216f5f3aac1c"},
+    {file = "virtualenv-20.4.6-py2.py3-none-any.whl", hash = "sha256:307a555cf21e1550885c82120eccaf5acedf42978fd362d32ba8410f9593f543"},
+    {file = "virtualenv-20.4.6.tar.gz", hash = "sha256:72cf267afc04bf9c86ec932329b7e94db6a0331ae9847576daaa7ca3c86b29a4"},
 ]
 wcwidth = [
     {file = "wcwidth-0.2.5-py2.py3-none-any.whl", hash = "sha256:beb4802a9cebb9144e99086eff703a642a13d6a0052920003a230f3294bbe784"},
diff --git a/pyproject.toml b/pyproject.toml
index 4161414..57ada26 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -48,6 +48,8 @@ bandit = "^1.6.2"
 black = "^21.4b2"
 blacken-docs = "^1.9.1"
 ipython = "^7.10.2"
+mdformat = "^0.6"
+mdformat-gfm = "^0.2"
 mypy = "^0.800"
 pre-commit = "^2.1.1"
 pre-commit-hooks = "^3.4.0"
@@ -60,9 +62,7 @@ sphinx = "^3.3.0"
 sphinx-autodoc-typehints = "^1.11.1"
 toml = "^0.10.1"
 tox = "^3.20.0"
-tox-poetry-installer = {extras = ["poetry"], version = "^0.7.0"}
-mdformat = "^0.6"
-mdformat-gfm = "^0.2"
+tox-poetry-installer = {extras = ["poetry"], version = "^0.8.0"}
 
 [build-system]
 requires = ["poetry-core>=1.0.0"]
diff --git a/tox.ini b/tox.ini
index 6375c8a..2214b1a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -11,6 +11,7 @@ skip_missing_interpreters = true
 [testenv]
 description = Run the tests
 require_locked_deps = true
+require_poetry = true
 locked_deps =
     pytest
     pytest-cov