checkers.utils.is_sys_guard: understands six.PY2 or six.PY3

Fixes pylint-dev#3501

doc/whatsnew/fragments/3501.feature

@@ -0,0 +1 @@
+Understand `six.PY2` and `six.PY3` for conditional imports.

pylint/checkers/utils.py

@@ -1831,6 +1831,11 @@ def is_sys_guard(node: nodes.If) -> bool:
             and value.as_string() == "sys.version_info"
         ):
             return True
+    elif (
+        isinstance(node.test, nodes.Attribute)
+        and node.test.as_string() in {'six.PY2', 'six.PY3'}
+    ):
+        return True
 
     return False
 

tests/checkers/unittest_utils.py

@@ -400,9 +400,19 @@ def test_if_sys_guard() -> None:
 
     if sys.some_other_function > (3, 8):  #@
         pass
+
+    import six
+    if six.PY2:  #@
+        pass
+
+    if six.PY3:  #@
+        pass
+
+    if six.something_else:  #@
+        pass
     """
     )
-    assert isinstance(code, list) and len(code) == 3
+    assert isinstance(code, list) and len(code) == 6
 
     assert isinstance(code[0], nodes.If)
     assert utils.is_sys_guard(code[0]) is True
@@ -412,6 +422,14 @@ def test_if_sys_guard() -> None:
     assert isinstance(code[2], nodes.If)
     assert utils.is_sys_guard(code[2]) is False
 
+    assert isinstance(code[3], nodes.If)
+    assert utils.is_sys_guard(code[3]) is True
+    assert isinstance(code[4], nodes.If)
+    assert utils.is_sys_guard(code[4]) is True
+
+    assert isinstance(code[5], nodes.If)
+    assert utils.is_sys_guard(code[5]) is False
+
 
 def test_if_typing_guard() -> None:
     code = astroid.extract_node(