pip install fastapi报错，AttributeError: module ‘typing‘ has no attribute ‘_ClassVar‘(包解决的)

环境：

错误如下：

    from ._parser import parse_requirement as _parse_requirement
  File "/usr/local/lib/python3.9/site-packages/pip/_vendor/packaging/_parser.py", line 12, in <module>
    from ._tokenizer import DEFAULT_RULES, Tokenizer
  File "/usr/local/lib/python3.9/site-packages/pip/_vendor/packaging/_tokenizer.py", line 12, in <module>
    class Token:
  File "/usr/local/lib/python3.9/site-packages/dataclasses.py", line 958, in dataclass
    return wrap(_cls)
  File "/usr/local/lib/python3.9/site-packages/dataclasses.py", line 950, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash, frozen)
  File "/usr/local/lib/python3.9/site-packages/dataclasses.py", line 800, in _process_class
    cls_fields = [_get_field(cls, name, type)
  File "/usr/local/lib/python3.9/site-packages/dataclasses.py", line 800, in <listcomp>
    cls_fields = [_get_field(cls, name, type)
  File "/usr/local/lib/python3.9/site-packages/dataclasses.py", line 659, in _get_field
    if (_is_classvar(a_type, typing)
  File "/usr/local/lib/python3.9/site-packages/dataclasses.py", line 550, in _is_classvar
    return type(a_type) is typing._ClassVar
AttributeError: module 'typing' has no attribute '_ClassVar'

解决办法1

pip uninstall dataclasses -y

解决方法2

rm -rf /usr/local/lib/python3.9/site-packages/dataclasses-0.6.dist-info \
rm -rf /usr/local/lib/python3.9/site-packages/dataclasses.py