tools/makemanifest.py: Use sys.executable when invoking Python scripts.

[Jongy]

Clean Ubuntu 18.04 doesn't even have a "python" program preinstalled, only "python3". So even if this script is py2 compatible, it won't run without additional installations.

Since everything in MicroPython is guaranteed to be py3 compatible, I think it's okay to explicitly state it whenever possible.

[dpgeorge]

This script was originally invoked via the makefile like $(PYTHON) make-frozen.py, so the Python version could be set globally using the PYTHON variable. But now this script is invoked via makemanifest.py. We could make makemanifest.py use the value of PYTHON to run these scripts ... but that's a makefile variable and doesn't seem available to makemanifest.py. Instead could use the Python interpreter that makemanifest.py itself was invoked with. That way the user can still use Python 2.

[Jongy]

Instead could use the Python interpreter that makemanifest.py itself was invoked with. That way the user can still use Python 2

Yeah I guess sys.executable should be used. It's just too much of a diff so I was trying to avoid it :)

The errors I get for a missing python binary are quite unclear: error freezing strings []: b'/usr/bin/env: \xe2\x80\x98python\xe2\x80\x99: No such file or directory\n' so I'd like to solve that.

[Jongy]

It's just too much of a diff

Well, I dunno why I thought this way.

[dpgeorge]

Great! Merged in b2dd443