black: Test failures with Python 3.9.0b1

Describe the bug

With Python 3.9.0b1, there are test failures:

======================================================================
FAIL: test_beginning_backslash (tests.test_black.BlackTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib64/python3.9/unittest/mock.py", line 1337, in patched
    return func(*newargs, **newkeywargs)
  File ".../black/tests/test_black.py", line 768, in test_beginning_backslash
    black.assert_equivalent(source, actual)
  File ".../black/src/black/__init__.py", line 6050, in assert_equivalent
    raise AssertionError(
AssertionError: INTERNAL ERROR: Black produced code that is not equivalent to the source.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: 
--- src
+++ dst
@@ -1,17 +1,25 @@
 Module(
   body=
-    Print(
-      dest=
-        None,  # NoneType
-      nl=
-        True,  # bool
-      values=
-        Constant(
-          kind=
-            None,  # NoneType
-          value=
-            b'hello, world',  # bytes
-        )  # /Constant
-    )  # /Print
+    Expr(
+      value=
+        Call(
+          args=
+            Constant(
+              kind=
+                None,  # NoneType
+              value=
+                'hello, world',  # str
+            )  # /Constant
+          func=
+            Name(
+              ctx=
+                Load(
+                )  # /Load
+              id=
+                'print',  # str
+            )  # /Name
+          keywords=
+        )  # /Call
+    )  # /Expr
   type_ignores=
 )  # /Module



======================================================================
FAIL: test_expression (tests.test_black.BlackTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../black/src/black/__init__.py", line 6029, in assert_equivalent
    src_ast = parse_ast(src)
  File ".../black/src/black/__init__.py", line 5948, in parse_ast
    return ast27.parse(src)
  File ".../black/__venv39__/lib64/python3.9/site-packages/typed_ast/ast27.py", line 50, in parse
    return _ast27.parse(source, filename, mode)
  File "<unknown>", line 1
    ...
    ^
SyntaxError: invalid syntax

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib64/python3.9/unittest/mock.py", line 1337, in patched
    return func(*newargs, **newkeywargs)
  File ".../black/tests/test_black.py", line 331, in test_expression
    black.assert_equivalent(source, actual)
  File ".../black/src/black/__init__.py", line 6031, in assert_equivalent
    raise AssertionError(
AssertionError: cannot use --safe with this file; failed to parse source file.  AST error message: invalid syntax (<unknown>, line 1)

======================================================================
FAIL: test_expression_diff (tests.test_black.BlackTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../black/tests/test_black.py", line 375, in test_expression_diff
    self.assertEqual(result.exit_code, 0)
AssertionError: 123 != 0

======================================================================
FAIL: test_expression_diff_with_color (tests.test_black.BlackTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../black/tests/test_black.py", line 403, in test_expression_diff_with_color
    self.assertIn("\033[1;37m", actual)
AssertionError: '\x1b[1;37m' not found in ''

======================================================================
FAIL: test_expression_ff (tests.test_black.BlackTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File ".../black/src/black/__init__.py", line 6029, in assert_equivalent
    src_ast = parse_ast(src)
  File ".../black/src/black/__init__.py", line 5948, in parse_ast
    return ast27.parse(src)
  File "<unknown>", line 1
    ...
    ^
SyntaxError: invalid syntax

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File ".../black/tests/test_black.py", line 362, in test_expression_ff
    black.assert_equivalent(source, actual)
  File ".../black/src/black/__init__.py", line 6031, in assert_equivalent
    raise AssertionError(
AssertionError: cannot use --safe with this file; failed to parse source file.  AST error message: invalid syntax (<unknown>, line 1)

----------------------------------------------------------------------
Ran 140 tests in 35.840s

FAILED (failures=5)

To Reproduce Steps to reproduce the behavior:

1. git clone psf/black && cd black
2. python3.9 -m venv __venv39__
3. . __venv39__/bin/activate
4. pip install -e '.[d]'
5. python -m unittest

Expected behavior is that tests should pass.

Environment (please complete the following information):

• Version: master (now a2408b3cb23fe252b3674cee484d742496bb3411)
• OS and Python version: Linux (Fedora 32 x86_64) / Python 3.9.0b1

Does this bug also happen on master? yes (but also on the latest version)