fix: support ast.Index, ast.ExtSlice in py3.8 when use subscript

[FLming]

in python3.8, like a[0], a[1:4:2, 1:4:2], is ast.Name + ast.Index / ast.ExtSlice.

case in tests/test_varname.py occurre error in py3.8.

E       varname.utils.ImproperUseError: Node 'x[Index]' detected, but only following nodes are supported: 
varname/utils.py:292: ImproperUseError
======================================================================================================= short test summary info =======================================================================================================
FAILED tests/test_varname.py::test_multi_vars_lhs - AssertionError: Regex pattern did not match.
FAILED tests/test_varname.py::test_subscript - varname.utils.ImproperUseError: Node 'x[Index]' detected, but only following nodes are supported: 

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[pwwang]

Thank you for your contribution. However, we have gently phased out support for Python 3.8. Only versions earlier than v0.11 are compatible with Python 3.8, and new features or fixes will not be applied to Python 3.8.