Fix unicode error

This fixes python-poetry#221 by exchanging the regex package for re (the former having fixed the long-standing bug in re (see [here](lark-parser/lark#590) for details). Not sure if this will work for poetry (i.e., whether adding regex as a dependency is acceptable)...but thought I would propose this change and wait for feedback to take it from there.
Thanks!

poetry/core/packages/package.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 import copy
-import re
+import regex as re
 
 from contextlib import contextmanager
 from typing import TYPE_CHECKING