CLI uses hard-coded binary to invoke project.py

[chrylis]

At line 2407, mbed.py uses a hard-coded exec call to 'python' to invoke project.py. As python is generally Python 3 on modern systems, this tends to break with syntax errors.

At line 2423, it uses python_cmd, which is presumably sane, and this is probably the right approach (barring an update of the whole CLI to Python 3, of course). Note that this idiom occurs elsewhere, such as at 2195.

[screamerbg]

@chrylis Thanks for the report. You're right, these should be unified.

[screamerbg]

@chrylis Implemented, merged and now available in Mbed CLI 1.2.2.