Due to bad typing of kwargs it things that env is a str

but in fact it in general should be a dict

jupyter_server/services/kernels/kernelmanager.py

@@ -246,7 +246,7 @@ async def _async_start_kernel(  # type:ignore[override]
             self.log.debug(
                 "Kernel args (excluding env): %r", {k: v for k, v in kwargs.items() if k != "env"}
             )
-            env = kwargs.get("env", None)
+            env = kwargs.get("env", None)  # type:ignore[unreachable]
             if env and isinstance(env, dict):
                 self.log.debug("Kernel argument 'env' passed with: %r", list(env.keys()))