python 3.9起，官方不支持在windows7及以下版本中安装和运行，为了不重装操作系统，经过尝试，找到了如下解决办法。

1.下载非安装版本 Windows embeddable package (64-bit)
https://www.python.org/ftp/python/3.9.1/python-3.9.1-embed-amd64.zip

2.解压缩文件到D:\python39，进入此目录，执行python，提示缺少api-ms-win-core-path-l1-1-0.dll，从网上下载了一个，将6.2.9200.16384版本放到解压缩目录中。
输出

D:\python39>python
Python path configuration:
  PYTHONHOME = (not set)
  PYTHONPATH = (not set)
  program name = 'python'
  isolated = 1
  environment = 1
  user site = 1
  import site = 0
  sys._base_executable = '\u0158\x08'
  sys.base_prefix = 'D:\\python39'
  sys.base_exec_prefix = 'D:\\python39'
  sys.platlibdir = 'lib'
  sys.executable = '\u0158\x08'
  sys.prefix = 'D:\\python39'
  sys.exec_prefix = 'D:\\python39'
  sys.path = [
    'D:\\python39',
    'D:\\python39',
  ]
Fatal Python error: init_fs_encoding: failed to get the Python codec of the filesystem encoding
Python runtime state: core initialized
ModuleNotFoundError: No module named 'encodings'

Current thread 0x0000058c (most recent call first):
<no Python frame>

3.下载源代码，
https://www.python.org/ftp/python/3.9.1/Python-3.9.1.tar.xz
再下载visual studio 2017,自己编译出python的二进制文件。不幸，也是同样的错误。

4.下载python 3.8非安装版本
https://www.python.org/ftp/python/3.8.7/python-3.8.7-embed-amd64.zip

5.可执行文件替换
将python 3.8的可执行文件python.exe解压缩，复制到D:\python39，替换原有的同名文件。 运行python，提示缺少python38.dll文件，将python39.dll重命名为python38.dll，python39.zip改名为python38.zip。
再次执行，成功。

D:\python39>python    
Python 3.9.1 (tags/v3.9.1:1e5d33e, Dec  7 2020, 17:08:21) [MSC v.1927 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.

测试3.9版新功能。 https://docs.python.org/zh-cn/3.9/whatsnew/3.9.html

>>> x = {"key1": "value1 from x", "key2": "value2 from x"}
>>> y = {"key2": "value2 from y", "key3": "value3 from y"}
>>> x | y
{'key1': 'value1 from x', 'key2': 'value2 from y', 'key3': 'value3 from y'}

也成功了。