pip21 报错 处理，降级

python3.6升级pip21.1后导致不兼容，

Traceback (most recent call last):
  File "C:\Program Files\Python36\Lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\Program Files\Python36\Lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\40122\Envs\py3_flask\Scripts\pip.exe\__main__.py", line 4, in <module>
  File "c:\users\40122\envs\py3_flask\lib\site-packages\pip\_internal\cli\main.py", line 9, in <module>
    from pip._internal.cli.autocompletion import autocomplete
  File "c:\users\40122\envs\py3_flask\lib\site-packages\pip\_internal\cli\autocompletion.py", line 10, in <module>
    from pip._internal.cli.main_parser import create_main_parser
  File "c:\users\40122\envs\py3_flask\lib\site-packages\pip\_internal\cli\main_parser.py", line 8, in <module>
    from pip._internal.cli import cmdoptions
  File "c:\users\40122\envs\py3_flask\lib\site-packages\pip\_internal\cli\cmdoptions.py", line 30, in <module>
    from pip._internal.utils.hashes import STRONG_HASHES
  File "c:\users\40122\envs\py3_flask\lib\site-packages\pip\_internal\utils\hashes.py", line 2, in <module>
    from typing import TYPE_CHECKING, BinaryIO, Dict, Iterator, List, NoReturn
ImportError: cannot import name 'NoReturn'

去官网下载get-pip.py包

使用管理员权限安装 python get-pip.py