We do not need the python module's find_installation() for this, as this
does various things to set up building and installing python modules
(pure python and C-API). This functionality is already tested in the
python tests.
Elsewhere, when we just need an interpreter capable of running python
scripts in order to guarantee a useful scripting language for custom
commands, it suffices to use find_program(), which does not run an
introspection script or do module imports, and is thus faster and
a bit cleaner.
Either way, both methods are guaranteed to find the python3 interpreter,
deferring to mesonlib.python_command for that guarantee.
test "71 summary" can sometimes return the python command with the
".exe" part all uppercased for mysterious Windows reasons. Smooth this
over with ExternalProgram.
Jussi Pakkanen
Eli Schwartz
Xavier Claessens