今天在windows上想升级一下pip，结果没升级成功，反而搞得pip用不了
    执行pip相关的命令时报下面的错误

    1. $ pip freeze
    2. Traceback (most recent call last):
    3.   File "d:\python3\lib\runpy.py", line 193, in _run_module_as_main  
    4.     "__main__", mod_spec)
    5.   File "d:\python3\lib\runpy.py", line 85, in _run_code
    6.     exec(code, run_globals)
    7.   File "D:\Python3\Scripts\pip.exe\__main__.py", line 5, in <module>
    8. ModuleNotFoundError: No module named 'pip'

    从网上查到解决办法
    1.先执行下面的命令
    python -m ensurepip
    2.再升级pip
    python -m pip install --upgrade pip