Updated shell for mkdir commands

[radarhere]

I recently pushed a commit to GitHub Actions. It failed for a perfectly valid reason, but when failing, test-windows reported -

The '-p' flag is meant to prevent this error, but I guess this doesn't work on Windows (https://en.wikipedia.org/wiki/Mkdir#Options)

So instead, this PR replaces 'mkdir' with a Python command. It also changes test.yml, just for symmetry.

See https://github.com/radarhere/Pillow/runs/1751747364?check_suite_focus=true for my failing case combined with this commit to produce a correct 'Prepare to upload errors'

[hugovk]

I wonder if it failed due to using the pwsh shell? Does it work when not specifying a shell? Or using another one?

https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#using-a-specific-shell

[radarhere]

If I do not specify the shell, then it fails the same.
if I specify the cmd shell, then it fails with "The syntax of the command is incorrect."
If I specify the bash shell, that does work... it seems odd to using bash on Windows, but if you think that is simpler, I can update this.

[hugovk]

If I specify the bash shell, that does work... it seems odd to using bash on Windows, but if you think that is simpler, I can update this.

Yes, let's go for this one. Thanks!

[nulano]

Checking the help documents for mkdir in cmd and powershell, I have found that mkdir <path> -Force works in PowerShell.

[hugovk]

Good to know, thanks! This'll do for now :)