brain: Add `__class_getitem__` to `subprocess.Popen` starting from Python 3.9

This is necessary for pylint to detect that `subprocess.Popen` is
subscriptable, starting from Python 3.9 (see PyCQA/pylint#4034).

    $ python3.9
    >>> import subprocess
    >>> subprocess.Popen.__class_getitem__
    <bound method GenericAlias of <class 'subprocess.Popen'>>

1 files changed, 7 insertions, 0 deletions

diff --git a/tests/unittest_brain.py b/tests/unittest_brain.py
index 7bb1f70f..4717ab76 100644
--- a/tests/unittest_brain.py
+++ b/tests/unittest_brain.py
@@ -1292,6 +1292,13 @@ class SubprocessTest(unittest.TestCase):
         assert isinstance(inferred, astroid.Const)
         assert isinstance(inferred.value, (str, bytes))
 
+    @test_utils.require_version("3.9")
+    def test_popen_does_not_have_class_getitem(self):
+        code = """import subprocess; subprocess.Popen"""
+        node = astroid.extract_node(code)
+        inferred = next(node.infer())
+        assert "__class_getitem__" in inferred
+
 
 class TestIsinstanceInference:
     """Test isinstance builtin inference"""