Log Message:
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Fix: Avoid access denied error on Windows.
This avoids the AppVeyor error "PermissionError: [WinError 5] Access is denied: 'C:\\Users\\appveyor\\AppData\\Local\\Temp\\1\\pip-bh_ifrdu-uninstall\\python35\\scripts\\pip.exe'"
This is probably caused by running the setuptools entry point for pip, which on Windows is pip.exe. Installing pip-accel tries to rewrite this, which (since it's currently executing) produces the error above.