Move StrEnum to models

This commit moves `StrEnum` to models
to prevent future cyclic import.

Author: code-yeongyu

aishell/models/language_model.py

@@ -1,6 +1,6 @@
 from enum import auto
 
-from aishell.utils import StrEnum
+from .str_enum import StrEnum
 
 
 class LanguageModel(StrEnum):

aishell/utils/str_enum.py renamed to aishell/models/str_enum.py

@@ -1,2 +1 @@
 from .make_executable_command import make_executable_command as make_executable_command
-from .str_enum import StrEnum as StrEnum