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

@@ -17,7 +17,6 @@
 import sys
 import time
 
-import _ast
 from setuptools import setup
 from setuptools.extension import Extension
 
@@ -50,9 +49,9 @@
     __version__ = [
         expr.value.s
         for expr in parsed_file.body
-        if isinstance(expr, _ast.Assign)
-        and isinstance(expr.targets[0], _ast.Name)
-        and isinstance(expr.value, _ast.Str)
+        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]