Replace optional [poetry] extra with explicit poetry dependencies

Fixes #79

tox.ini

@@ -6,8 +6,6 @@ skip_missing_interpreters = true
 description = Run the tests
 require_locked_deps = true
 require_poetry = true
-extras =
-    poetry
 locked_deps =
     pytest
     pytest-cov
@@ -82,8 +80,7 @@ commands =
       --format requirements.txt \
       --output {envtmpdir}/requirements.txt \
       --without-hashes \
-      --with dev \
-      --extras poetry
+      --with dev
     safety check \
       --file {envtmpdir}/requirements.txt \
       --output text \