matplotlib现在已经发布了python 3.9的***，因此 pip install --upgrade matplotlib 应该可以工作.

edit: matplotlib has now released wheels for python 3.9 so pip install --upgrade matplotlib should work.

原始答案

matplotlib尚未为3.9版做好准备，因此您的python尝试从源代码进行构建.您应该降级到 python 3.8，然后一切正常

matplotlib hasn't made a wheel yet for version 3.9 so your python attempted to build it from source. You should downgrade to python 3.8 and then everything should work