validate package scoping causes incorrect package namespacing

[majuscule]

I believe that I have found a bug in the configobj/validate interaction. I am not entirely sure if this code is causing a user-facing issue, but I am fairly certain that the code below is not behaving as intended.

https://github.com/DiffSK/configobj/blob/master/src/configobj/__init__.py#L2177
excerpt:

if not preserve_errors or isinstance(cause, self._vdtMissingValue):

The import of validate (init.py#L2142) is relative, and the resulting classes are namespaced within configobj: <class 'configobj.validate.VdtMissingValue'> - causing the above test to fallthrough.

This is causing additional problems when I attempt to raise VdtMissingDefinition (PR
#127) from configobj, as the resulting preserve_errors dictionary contains exceptions with differing namespaces, forcing awkward imports from external code which would like to test entry error types.

[jun66j5]

_version.py has the same problem, violates packaging namespace. It should be configobj/_version.py.

[robdennis]

I think this is addressed in 5.0.7, but since I needed to manually implement the fix, please reopen if I messed it up