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 BoundValidators callable class
  • Significantly simplified ValidatorManager implementation.
• Added test case to _tests/test_asdf.py to verify the limited pickling that is now supported.

[braingram]

How do the changes in this file relate to this PR? If unrelated would you split them into a different PR?

[sydduckworth]

Changing get_jsonschema_validators was necessary to fix the pickling issue.
Because of that change, validate became both unused and untested, so I removed it.
Removing validate allowed me to clean up the constructor for ValidatorManager.

I renamed get_jsonschema_validators to just validators because it is now the only thing that ValidatorManager actually returns, and I found the jsonschema part confusing since it isn't really actually related to jsonschema at all (or any more than the input validators are).

[sydduckworth]

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

Once #2040 is merged I'm going to add @deprecated to ValidatorManager.get_jsonschema_validators.

The issue I'm now running into is that ValidatorManager.validate isn't actually being used or tested anywhere, which is causing the code coverage to drop.
So I could add unit tests for that function, or I could mark it as deprecated and add a coverage exception for deprecated functions?