Re-added install_sdp_python.sh as an Unsupported script.
Updates to install_sdp_python.sh v1.5.0 from prior version:
* Updated to be compatible with non-backward-compatible changes in
the P4Python setup.py, using 'setup.cfg' rather than '--apidir'
* Default P4API version is r21.1.
* Default Python version is 3.9.6.
* Improvements making debugging of build failures easier by capturing
more data in the main log, to avoid having to follow separate log files
listed in the main log.