Expects python to be called `python` and not e.g. `python3`

[Zentrik]

Assumes python is invoked by calling python at

faber/src/faber/tools/python.py

Line 51 in 11d443b

self.command = command or 'python'

and this error seems to be suppressed by default at

faber/src/faber/tool.py

Lines 149 to 155 in 11d443b

	except Exception as e:
	from . import debug
	logger.info('trying to instantiate {} yields "{}"'
	.format(cls.__name__, e))
	if debug:
	import traceback
	traceback.print_exc()

I encountered this using faber 0.5.3 from PyPi and trying to build boost python.