> running python.exe results in an error message popup which tells me ...
> that the "api-ms-win-core-path-l1-1-0.dll" is missing on the computer. 
> I googled for it and it seems that this dll is not a part of Windows 7.

Microsoft ended extended support for Windows 7 back in January of this year, so Python 3.9 does not support Windows 7, in accordance with PEP 11 [1]. I'd expect the installer to fail on unsupported versions of Windows.

[1]: https://www.python.org/dev/peps/pep-0011/#microsoft-windows