Doctest needs to Ignore objects that inspect.unwrap throws ValueError due to "infinite" nesting.

This case is relatively commonly reproduced by importing unittest.mock.call into a module's namespace.

The following code produces a module that will crash doctest:

from unittest.mock import call

This is due to the call to unwrap throwing an uncaught ValueError.

For this case we can simply ignore the function and not attempt to extract docstring from this function.

This diff is intentionally done as a branch with two commits to provide failing test case and then a fix to show the problem is contained.

Future thoughts:

1. Should catching this be optional defaulting to true, but allowing for folks to rethrow the exception to insist on truly clean code?
2. Should we break out _find() into more sub functions so that we can more easily derive our own versions for each sub function that _find does making it easier to fix future issues?
3. Should we include a way to denylist by id some objects so that future objects that cause problems with doctest can be passed in to ignore?

Note: This branch diff will need to be pulled up into 3.9 and master as the bug exists in those branches as well.

https://bugs.python.org/issue35753