BUG: Fix install path for python-real

Sam Horvathrequested to merge
github/fork/cquammen/4553-python-real-path into master

Created by: cquammen

python-real is inside the same directory as SlicerPython, so the installation subdirectory should be . instead of Slicer_BIN_DIR.

Fixes https://issues.slicer.org/view.php?id=4553.

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