scan-build-py: Change scripts to explicitly require python3

The "#!" line in all scan-build-py scripts were using just bare
"/usr/bin/python" which according to PEP-0394 can be either python3,
python2 or not exist at all.

E.g in latest debian and ubuntu releases "/usr/bin/python" does not
exist at all by default and user must install python-is-python2 or
python-is-python3 packages to get the bare version less "python"
command.

Until recently (70b06fe8 "scan-build-py: Force the opening in utf-8"
changed "libscanbuild") these scripts worked in both python2 and
python3, but now they (rightfully) are python3 only, and broke on
systems where the "python" command means python2.

By changing the "#!" to be "python3" it is not only explicit that the
scripts require python3 it also works on systems where "python" command
is python2 or nonexistent.

Differential Revision: https://reviews.llvm.org/D126804