Could not find a version that satisfies the requirement python==3.8

对于错误信息"Could not find a version that satisfies the requirement python==3.8"，这是因为Python解释器本身不需要通过pip安装，而是需要从官方网站下载安装。你可以在Python官网上下载适合你操作系统的Python 3.8版本并进行安装。

另外，对于警告信息"WARNING: You are using pip version 20.2.3; however, version 21.0.1 is available. You should consider upgrading via the 'c:\program files\python38\python.exe -m pip install --upgrade pip' command."，这是因为你的pip版本过旧，建议你使用命令行运行"python -m pip install --upgrade pip"来升级pip到最新版本。