appdata/local/microsoft/windowsapps/python: permission denied
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As far as I can tell, this was caused by a conflict with the version of Python 3.7 that was recently added into the Windows Store. It looks like this added two "stubs" called python.exe and python3.exe into the %USERPROFILE%\AppData\Local\Microsoft\WindowsApps folder, and in my case, this was inserted before my existing Python executable's entry in the PATH.