[pythonic resources] only import pydantic core if using pydantic 2 (#…

…17589)

## Summary

Pydantic 2 introduces a `pydantic_core` module which holds some core
imports separate from the main module. Theoretically, a user should only
have it installed if using `pydantic>=2.0.0`, but it may happen that
version requirement pins lead a user to first install `pydantic>=2.0.0`
(and `pydantic_core`) and then later downgrade to `pydantic<2.0.0`.

Dagster will misbehave in this case, since we would use the
`pydantic>=2.0.0` notion of 'undefined' from `pydantic_core` in this
case instead of `None`. This change makes us more strictly use this
import if and only if pydantic 2 is being used.

## Test Plan

Manually constructed environment locally w/ `pydantic<2.0.0` and
`pydantic-core`, ran tests which failed. Ran tests with change in this
PR, tests succeed.

python_modules/dagster/dagster/_config/pythonic_config/pydantic_compat_layer.py

@@ -14,19 +14,18 @@
     IAttachDifferentObjectToOpContext as IAttachDifferentObjectToOpContext,
 )
 
+USING_PYDANTIC_2 = int(pydantic.__version__.split(".")[0]) >= 2
+
 PydanticUndefined = None
-try:
+if USING_PYDANTIC_2:
     from pydantic_core import PydanticUndefined as _PydanticUndefined  # type: ignore
 
     PydanticUndefined = _PydanticUndefined
-except:
-    pass
+
 
 if TYPE_CHECKING:
     from pydantic.fields import ModelField
 
-USING_PYDANTIC_2 = int(pydantic.__version__.split(".")[0]) >= 2
-
 
 class ModelFieldCompat:
     """Wraps a Pydantic model field to provide a consistent interface for accessing