• cattrs version: 1.7.0
• Python version: 3.9.0
• Operating System: Ubuntu Linux

Description

cattrs 1.7.0 introduced support for Literals. Therefore it imports on Python 3.9 some classes part of the typing module. For instance the _LiteralGenericAlias is imported:
https://github.com/Tinche/cattrs/blob/71a7ed2b4c72558af77487ae991f19cc5401c378/src/cattr/_compat.py#L195

These classes weren't introduced with the first release of Python 3.9.0 but with Python 3.9.1. See comparison between Python 3.9.0 and 3.9.1: python/cpython@v3.9.0...v3.9.1

The conditional import in the _compat.py module doesn't check the patch version of Python 3.9.x:
https://github.com/Tinche/cattrs/blob/71a7ed2b4c72558af77487ae991f19cc5401c378/src/cattr/_compat.py#L88
and:
https://github.com/Tinche/cattrs/blob/71a7ed2b4c72558af77487ae991f19cc5401c378/src/cattr/_compat.py#L181

What I Did

Import cattrs in user code on Python 3.9.0.

  File \"mycode.py\", line 7, in <module>
    import cattr
  File \"venv/lib/python3.9/site-packages/cattr/__init__.py\", line 1, in <module>
    from .converters import Converter, GenConverter, UnstructureStrategy
  File \"venv/lib/python3.9/site-packages/cattr/converters.py\", line 10, in <module>
    from ._compat import (
  File \"venv/lib/python3.9/site-packages/cattr/_compat.py\", line 191, in <module>
    from typing import (
ImportError: cannot import name '_LiteralGenericAlias' from 'typing' (/usr/lib/python3.9/typing.py)