MNT: use public ast module in setup.py

The `Str` object is no longer in in `_ast` is py38, switch
to using the public `ast` module instead.

python/cpython#9445
https://bugs.python.org/issue32892

setup.py

@@ -16,7 +16,6 @@
 import time
 from pathlib import Path
 
-import _ast
 from setuptools import setup
 from setuptools.extension import Extension
 
@@ -36,8 +35,8 @@
 __version__ = [
     expr.value.s
     for expr in parsed_file.body
-    if isinstance(expr, _ast.Assign)
-    and isinstance(expr.targets[0], _ast.Name)
+    if isinstance(expr, ast.Assign)
+    and isinstance(expr.targets[0], ast.Name)
     and isinstance(expr.value, _ast.Str)
     and expr.targets[0].id == "__version__"
 ][0]