pip install --upgrade pip 更新pip的最新版本，系统却会尝试下载pip包的所有版本。

macOS Big Sur系统，使用brew安装的python3.9下，使用pip install --upgrade pip 更新pip的最新版本，系统却会尝试下载pip包的所有版本并 Installing build dependencies，更新其他的包也类似。输出如下：

chenweiqiang:lib chweiqiang$ pip3 install –upgrade pip
Looking in indexes: pypi.tuna.tsinghua.edu.cn/simple
Requirement already satisfied: pip in ./python3.9/site-packages (20.3.2)
Collecting pip
Downloading pypi.tuna.tsinghua.edu.cn/packages... (1.5 MB)
|████████████████████████████████| 1.5 MB 163 kB/s
Downloading pypi.tuna.tsinghua.edu.cn/packages... (1.5 MB)
|████████████████████████████████| 1.5 MB 520 kB/s
Installing build dependencies … done
Getting requirements to build wheel … done
Preparing wheel metadata … done
Downloading pypi.tuna.tsinghua.edu.cn/packages... (1.5 MB)
|████████████████████████████████| 1.5 MB 349 kB/s
Downloading pypi.tuna.tsinghua.edu.cn/packages... (1.5 MB)
|████████████████████████████████| 1.5 MB 128 kB/s
Installing build dependencies … done
Getting requirements to build wheel … done
Preparing wheel metadata … done
Downloading pypi.tuna.tsinghua.edu.cn/packages... (1.5 MB)
|████████████████████████████████| 1.5 MB 513 kB/s
Downloading pypi.tuna.tsinghua.edu.cn/packages... (1.5 MB)
|████████████████████████████████| 1.5 MB 443 kB/s
Installing build dependencies … done
Getting requirements to build wheel … done
Preparing wheel metadata … done
Downloading pypi.tuna.tsinghua.edu.cn/packages... (1.5 MB)
|████████████████████████████████| 1.5 MB 160 kB/s
Downloading pypi.tuna.tsinghua.edu.cn/packages... (1.5 MB)
|████████████████████████████████| 1.5 MB 515 kB/s
Installing build dependencies … done