Fix AsdfFile pickling for instances without file descriptors

[sydduckworth]

Description

This PR fixes two bugs that prevented AsdfFile instances from being pickled, which partially addresses #1782.
The result is that now asdf files can be pickled if they don't contain a file descriptor, which basically means files that were created from an in-memory dict.

• Fixed a bug in which AsdfObject instances were failing to deserialize because of their conflicting dict and UserDict base classes. Added a manual __reduce__ override which resolves the problem.
• Updated ValidatorManager to not return local callables defined inside a method, which can't be pickled.
  • ValidatorManager now returns a new JsonSchemaValidators callable class
  • Significantly simplified ValidatorManager implementation.
• Added test case to _tests/test_asdf.py to verify the limited pickling that is now supported.

[sydduckworth]

@braingram I've reverted the public API for ValidatorManager.

[braingram]

I think we'd benefit from including a wider set of possible AsdfFile instances for initial pickling support.

Testing this PR if validate is called on an AsdfFile (created from an in-memory tree) prior to pickling the pickling fails with:

import asdf, pickle, numpy as np
af = asdf.AsdfFile({"arr": np.arange(42)})
pickle.loads(pickle.dumps(af))  # passes
af.validate()
pickle.loads(pickle.dumps(af))  # fails

TypeError: cannot pickle 'weakref.ReferenceType' object

The same error appears if array storage type is set on "arr" prior to pickling.

af.set_array_storage(af["arr"], "inline")
pickle.loads(pickle.dumps(af))  # fails

As part of this we should address what operations cause pickling to fail and why? What are the "typical" AsdfFile instances we want to support (probably at least a tree created from in-memory objects but also one read from a file and read then modified)?

I think we shouldn't claim pickling support until we support whatever we decide are the "typical" cases.

[sydduckworth]

Is there a different changelog entry you would prefer for a PR that specifically fixes the two bugs addressed here but that does not fully enable pickling?