c.KubeSpawner.environment not interpreting callables

[Khagesh16]

Bug description

This change is causing an issue when the environment accepts the callable function. https://github.com/jupyterhub/kubespawner/pull/759

This same piece of logic used to be working on version 5.0.0 but failing on the latest version.

How to reproduce

This is a sample dictionary which we are passing to c.KubeSpawner.environment with other environment variables.

{
    "JUPYTERHUB_SINGLEUSER_APP": "jupyter_server.serverapp.ServerApp",
    "USERNAME": lambda spawner: str(spawner.user.name),
    "JUPYTER_ENABLE_LAB": "yes",
    "AWS_STS_REGIONAL_ENDPOINTS": "regional",
}

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behaviour

It should replace the username with actual username

{
    "CPU_LIMIT": "3.0",
    "CPU_GUARANTEE": "2.0",
    "JUPYTERHUB_SINGLEUSER_APP": "jupyter_server.serverapp.ServerApp",
    "USERNAME": "kpandya",
    "JUPYTER_ENABLE_LAB": "yes"}

Actual behaviour

{"CPU_LIMIT": "3.0",
    "CPU_GUARANTEE": "2.0",
    "JUPYTERHUB_DEFAULT_URL": "/lab",
    "JUPYTERHUB_SINGLEUSER_APP": "jupyter_server.serverapp.ServerApp",
    "USERNAME": < function < lambda > at 0x7ff12b035120 > ,
    "JUPYTER_ENABLE_LAB": "yes"}

Your personal set up

• OS:
• Version(s):

Full environment

``` # paste output of `pip freeze` or `conda list` here ```

Configuration

```python # jupyterhub_config.py ```

Logs

``` # paste relevant logs here, if any ```

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[jabbera]

This is a dupe of https://github.com/jupyterhub/kubespawner/issues/815