Use ast.Constant for recent Python versions (#526)

ast.Str is planned to removed in Python 3.14, use ast.Constant instead
whenever the later is available.

fire/parser.py

@@ -20,7 +20,12 @@
 
 import argparse
 import ast
+import sys
 
+if sys.version_info[0:2] < (3, 8):
+  _StrNode = ast.Str
+else:
+  _StrNode = ast.Constant
 
 def CreateParser():
   parser = argparse.ArgumentParser(add_help=False)
@@ -127,4 +132,4 @@ def _Replacement(node):
   # These are the only builtin constants supported by literal_eval.
   if value in ('True', 'False', 'None'):
     return node
-  return ast.Str(value)
+  return _StrNode(value)