pip问题：WARNING: You are using pip version 19.2.3, however version 21.0.1 is available.

1. The questions are as follows:

The specific error message is:

2. The solution is as follows:

step 1:

(1) WIN+R, enter cmd, and bring up the command box:

(2) Enter the statement "pip3 install -index-url https://pypi.douban.com/simple requests" and execute it, the result is:

Step 2:

Enter the statement "python -m pip install --upgrade pip --index-url https://pypi.douban.com/simple requests" and execute it, the result is:

Step 3: Result: Re-enter the statement you have encountered a problem, for example: pip install flask, the result is as follows:

3. Congratulations, the problem is solved!