自升级到版本20.0.2后，无法将软件包安装到虚拟环境

Question

我最近将点更新为版本20.0.2，并且我目前正在虚拟环境中的项目上工作，无法将任何软件包安装到虚拟环境中。我尝试在普通命令行上安装它，虽然安装成功，但是在虚拟环境中一直出现错误。

这是我尝试的命令：

(11_env) C:\Users\user\Desktop\learning_log>pip install django

这是我不断收到的错误消息：

Traceback (most recent call last):
  File "C:\Users\user\AppData\Local\Programs\Python\Python37\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\Users\user\AppData\Local\Programs\Python\Python37\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\user\Desktop\learning_log\11_env\Scripts\pip.exe\__main__.py", line 7, in <module>
  File "c:\users\user\desktop\learning_log\11_env\lib\site-packages\pip\_internal\cli\main.py", line 73, in main
    command = create_command(cmd_name, isolated=("--isolated" in cmd_args))
  File "c:\users\user\desktop\learning_log\11_env\lib\site-packages\pip\_internal\commands\__init__.py", line 96, in create_command
    module = importlib.import_module(module_path)
  File "C:\Users\user\AppData\Local\Programs\Python\Python37\lib\importlib\__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 728, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "c:\users\user\desktop\learning_log\11_env\lib\site-packages\pip\_internal\commands\install.py", line 23, in <module>
    from pip._internal.cli.req_command import RequirementCommand
  File "c:\users\user\desktop\learning_log\11_env\lib\site-packages\pip\_internal\cli\req_command.py", line 17, in <module>
    from pip._internal.index import PackageFinder
ImportError: cannot import name 'PackageFinder' from 'pip._internal.index' (c:\users\user\desktop\learning_log\11_env\lib\site-packages\pip\_internal\index\__init__.py)

注意：在升级Pip版本之前，我已经在虚拟环境中安装了软件包。

谢谢。