Fix circular import: use submodule import for C extension

core.py and simple.py used  which triggered
re-execution of __init__.py during package initialization. Changed to
 which imports the C extension submodule
directly without requiring full parent package resolution.

This fixes the ImportError seen in build-sdist CI when running
 after installation.

Author: neluca

tinybpe/core.py

@@ -11,7 +11,7 @@
 
 import regex as re
 
-from tinybpe import bpe
+import tinybpe.bpe as bpe
 from tinybpe._model_io import BPEParam, save_bpe_model, save_bpe_vocab
 
 

tinybpe/simple.py

@@ -8,7 +8,7 @@
 
 from typing import Callable
 
-from tinybpe import bpe
+import tinybpe.bpe as bpe
 from tinybpe._model_io import save_bpe_model