return the full path of 'python' executable if found.
See also
Other executable full path:
air_exec(),
chrome_exec(),
excel_exec(),
firefox_exec(),
libreoffice_exec(),
node_exec(),
npm_exec(),
pandoc_exec(),
pip_exec(),
powerpoint_exec(),
word_exec()
Examples
if(exec_available("python"))
message(python_exec())
#> /usr/bin/python3