When a program located by find_program is executable directly, meson
will execute it. If it is a script, its shebang (#!) line will control
which interpreter is used. tempita.py specifies a standard shebang line
of #!/usr/bin/env python3, so that python3 can be located by PATH
lookup.

In cases where python3 as found in the PATH is not correct, such as on
systems that have multiple copies of python3 installed when building
numpy for one that is not the python3 found by searching PATH, this
interpreter will be wrong. When the python3 found by this mechanism
doesn't have Cython available, it will cause the numpy build to fail.

With the executable bit removed from tempita.py, meson will scan the
script when attempting to execute it, locating the shebang line and
substituting the appropriate sys.executable. See
https://github.com/numpy/meson/blob/22df45a31981874310a78dde0df59a6a7c5ebb29/mesonbuild/programs.py#L179.