add pytest to docs env, because this resolves a warning about a missing pytest import?

Author: d-v-b

pyproject.toml

@@ -109,7 +109,8 @@ docs = [
     'numcodecs[msgpack]',
     'rich',
     's3fs>=2023.10.0',
-    'astroid<4'
+    'astroid<4',
+    'pytest'
 ]