Fix, test and support Python 3.9

lib/astunparse/printer.py

@@ -4,6 +4,9 @@
 import six
 
 
+_NOPE = object()
+
+
 class Printer(ast.NodeVisitor):
 
     def __init__(self, file=sys.stdout, indent="  "):
@@ -19,6 +22,7 @@ def write(self, text):
         self.f.write(six.text_type(text))
 
     def generic_visit(self, node):
+        cls = type(node)
 
         if isinstance(node, list):
             nodestart = "["
@@ -27,7 +31,8 @@ def generic_visit(self, node):
         else:
             nodestart = type(node).__name__ + "("
             nodeend = ")"
-            children = [(name + "=", value) for name, value in ast.iter_fields(node)]
+            children = [(name + "=", value) for name, value in ast.iter_fields(node)
+                        if not (value is None and getattr(cls, name, _NOPE) is None)]
 
         if len(children) > 1:
             self.indentation += 1

setup.py

@@ -52,6 +52,7 @@ def read_version():
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Topic :: Software Development :: Code Generators',
     ],
     test_suite='tests',

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py35, py36, py37, py38
+envlist = py27, py35, py36, py37, py38, py39
 
 [testenv]
 usedevelop = True