diff --git a/.github/workflows/persubmit.yml b/.github/workflows/persubmit.yml
index f598a7a2cf7b6..78606899cc57f 100644
--- a/.github/workflows/persubmit.yml
+++ b/.github/workflows/persubmit.yml
@@ -16,8 +16,8 @@ jobs:
           - os: macos-latest
             python: 3.7
             with_cc: OFF
-          - os: ubuntu-16.04  # TODO(archibate): windows-latest
-            python: 3.8
+          - os: ubuntu-latest
+            python: 3.9
             with_cc: OFF
           - os: ubuntu-latest
             python: 3.8
diff --git a/python/taichi/lang/transformer.py b/python/taichi/lang/transformer.py
index e7cc3ccc245b4..8d27e5cc52963 100644
--- a/python/taichi/lang/transformer.py
+++ b/python/taichi/lang/transformer.py
@@ -220,7 +220,8 @@ def visit_Assign(self, node):
 
             def tuple_indexed(i):
                 indexing = self.parse_stmt('__tmp_tuple[0]')
-                indexing.value.slice.value = self.parse_expr("{}".format(i))
+                self.set_subscript_index(indexing.value,
+                                         self.parse_expr("{}".format(i)))
                 return indexing.value
 
             for i, target in enumerate(targets):
@@ -565,15 +566,32 @@ def visit_For(self, node):
             else:  # Struct for
                 return self.visit_struct_for(node, is_grouped=False)
 
+    @staticmethod
+    def get_subscript_index(node):
+        assert isinstance(node, ast.Subscript), type(node)
+        # ast.Index has been deprecated in Python 3.9,
+        # use the index value directly instead :)
+        if isinstance(node.slice, ast.Index):
+            return node.slice.value
+        return node.slice
+
+    @staticmethod
+    def set_subscript_index(node, value):
+        assert isinstance(node, ast.Subscript), type(node)
+        if isinstance(node.slice, ast.Index):
+            node.slice.value = value
+        else:
+            node.slice = value
+
     def visit_Subscript(self, node):
         self.generic_visit(node)
 
         value = node.value
-        indices = node.slice
-        if isinstance(indices.value, ast.Tuple):
-            indices = indices.value.elts
+        indices = self.get_subscript_index(node)
+        if isinstance(indices, ast.Tuple):
+            indices = indices.elts
         else:
-            indices = [indices.value]
+            indices = [indices]
 
         call = ast.Call(func=self.parse_expr('ti.subscript'),
                         args=[value] + indices,
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000000000..59b7fd8c6524c
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1,14 @@
+setuptools
+astor
+pybind11
+pylint
+sourceinspect
+pytest
+pytest-rerunfailures
+pytest-xdist
+yapf
+numpy
+GitPython
+coverage
+colorama
+autograd