Revert "config update (#967)" (#968)

This reverts commit 82331b7.

Author: jmmshn

mp_api/client/core/settings.py

@@ -3,7 +3,7 @@
 from typing import List
 
 from pydantic import Field
-from pydantic_settings import SettingsConfigDict, BaseSettings
+from pydantic_settings import BaseSettings
 from pymatgen.core import _load_pmg_settings
 
 from mp_api.client import __file__ as root_dir
@@ -26,8 +26,6 @@ class MAPIClientSettings(BaseSettings):
     python module.
     """
 
-    model_config = ConfigDict(env_prefix="MPRESTER_")
-
     TEST_FILES: str = Field(
         os.path.join(os.path.dirname(os.path.abspath(root_dir)), "../../test_files"),
         description="Directory with test files",
@@ -88,4 +86,6 @@ class MAPIClientSettings(BaseSettings):
     MAX_LIST_LENGTH: int = Field(
         _MAX_LIST_LENGTH, description="Maximum length of query parameter list"
     )
-    model_config = SettingsConfigDict(env_prefix="MPRESTER_")
+
+    class Config:
+        env_prefix = "MPRESTER_"