Use executable from sys module instead of hardcoded python

[frenzymadness]

Hello.

I met this issue when trying to run tests of doit module on a Python3-only system. Problem is that on a lot of systems python still means Python 2 and when you have only Python 3 installed, there is no python command at all.

I think that the best will be to execute commands inside test suite under the same Python as test suite itself.

This fix will also enable to run the test suite with various Python versions.

What do you think?

[coveralls]

Coverage increased (+0.0002%) to 99.73% when pulling 35efd05 on frenzymadness:python-executable-in-tests into 40c9043 on pydoit:master.

[schettino72]

nice. I didnt know this sys.executable existed 😁