python/python_preamble_basic/python_preamble_basic: unset PYTHON_PREAMBLE_PYTHON_MAJOR_VERSION
python/python_preamble_basic/python_preamble_basic: unset PYTHON_PREAMBLE_PYTHON_MAJOR_VERSION

Save off PYTHON_PREAMBLE_PYTHON_MAJOR_VERSION into a temporary
variable.  Then unset the original
PYTHON_PREAMBLE_PYTHON_MAJOR_VERSION variable.  Then, if/when this
script executes child processes that, in turn, reuse this same
python_preamble_basic script, those child processes can opt to use a
_different_ PYTHON_PREAMBLE_PYTHON_MAJOR_VERSION value without
interference from the parent processes.

bgoodr/how-to,bgoodr/how-to