do not use global install of poetry in tox[mypy]

tox.ini

@@ -25,8 +25,11 @@ deps =
 commands = {env:_PYTEST_CMD:pytest} {env:_PYTEST_MORE_ARGS:} {posargs:-vvl}
 
 [testenv:mypy]
-skip_install = true
-allowlist_externals = poetry
+deps = poetry
+allowlist_externals = sh
 commands_pre =
-    poetry install --with=dev
+    sh -c "\
+      poetry export --only=dev --format requirements.txt > requirements.txt && \
+      pip install -r requirements.txt && \
+      :"
 commands = mypy