Cannot pickle `jinja2.utils.missing`

[mattclay]

The jinja2.utils.missing singleton instance cannot be pickled since pickle cannot find MissingType due to the jinja2.utils.MissingType attribute not existing:

import pickle
import jinja2.utils

pickle.dumps(jinja2.utils.missing)

Running the above code results in the following error:

Traceback (most recent call last):
  File "/tmp/bug.py", line 4, in <module>
    pickle.dumps(jinja2.utils.missing)
_pickle.PicklingError: Can't pickle <class 'jinja2.utils.MissingType'>: attribute lookup MissingType on jinja2.utils failed

Attempting to pickle (and then restore) the jinja2.utils.missing singleton instance should succeed. Supporting this would allow pickling of jinja2.runtime.Undefined instances which do not set obj, once https://github.com/pallets/jinja/issues/2025 is also resolved.

Environment:

• Python version: 3.11.1
• Jinja version: 3.1.4