Used UPX to reduce Windows x64 build size by ~23MB (~23%) #332
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Also improved some scripts relative paths whilst I was here.
UPX is only used by PyInstaller on Windows: https://pyinstaller.org/en/stable/usage.html#using-upx
UPX doesn't really support ARM64: https://github.com/upx/upx/issues?q=is%3Aissue%20is%3Aopen%20arm64