Fix breaking Python 3.9.0a5.

Fixes #193.

docs/CHANGES.rst

@@ -1,12 +1,16 @@
 Changes
 =======
 
-5.1 (unreleased)
-----------------
+5.1a0 (unreleased)
+------------------
 
 - Drop install dependency on ``setuptools``.
   (`#189 <https://github.com/zopefoundation/RestrictedPython/issues/189>`_)
 
+- Start supporting Python 3.9.0a5. (Be aware that this support currently only
+  makes RestrictedPython usable on Python 3.9. The security implications of
+  using Python 3.9 have not yet been checked.)
+
 
 5.0 (2019-09-03)
 ----------------

setup.py

@@ -32,7 +32,7 @@ def read(*rnames):
 
 setup(
     name='RestrictedPython',
-    version='5.1.dev0',
+    version='5.1a0.dev0',
     url='https://github.com/zopefoundation/RestrictedPython',
     license='ZPL 2.1',
     description=(
@@ -70,7 +70,7 @@ def read(*rnames):
     package_dir={'': 'src'},
     install_requires=[
     ],
-    python_requires=">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, <3.9",  # NOQA: E501
+    python_requires=">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, <3.10",  # NOQA: E501
     tests_require=tests_require,
     extras_require={
         'test': tests_require,

src/RestrictedPython/transformer.py

@@ -355,6 +355,9 @@ def transform_slice(self, slice_):
         if isinstance(slice_, ast.Index):
             return slice_.value
 
+        elif isinstance(slice_, ast.Name):  # Python 3.9.0a5+ for a[b]
+            return slice_
+
         elif isinstance(slice_, ast.Slice):
             # Create a python slice object.
             args = []

tests/transformer/test_subscript.py

@@ -17,6 +17,23 @@ def test_read_simple_subscript(mocker):
     assert (value, 'b') == glb['simple_subscript'](value)
 
 
+VAR_SUBSCRIPT = """
+def simple_subscript(a, b):
+    return a[b]
+"""
+
+
+def test_read_subscript_with_variable(mocker):
+    value = [1]
+    idx = 0
+    _getitem_ = mocker.stub()
+    _getitem_.side_effect = lambda ob, index: (ob, index)
+    glb = {'_getitem_': _getitem_}
+    restricted_exec(VAR_SUBSCRIPT, glb)
+
+    assert (value, 0) == glb['simple_subscript'](value, idx)
+
+
 TUPLE_SUBSCRIPTS = """
 def tuple_subscript(a):
     return a[1, 2]