无法在python virtualenv中使用pip

Question

我创建了一个名为proj_env的virtualenv，并在Windows的命令提示符中将其激活，如下所示：

C:\Users\me\Desktop\loan_predictor\proj_env\Scripts>activate.bat

(proj_env) C:\Users\me\Desktop\loan_predictor\proj_env\Scripts>pip3 install flask

Traceback (most recent call last):
  File "D:\Python\Python35\Lib\runpy.py", line 184, in _run_module_as_main
    "__main__", mod_spec)
  File "D:\Python\Python35\Lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\me\Desktop\loan_predictor\proj_env\Scripts\pip3.exe\__main__.py", line 5, in <module>
ImportError: No module named 'pip._internal'

我不确定pip和pip3为何都不起作用，尽管在我执行pip3 install virtualenv时起作用。

我想念什么？