Python checker says something 'is' python when calling guess_contains

[terriko]

There's a bug on line 69:

https://github.com/intel/cve-bin-tool/blob/master/cve_bin_tool/checkers/python.py#L69

It's guessing if it contains python, but then returns that it "is" python. it should be setting that value to "contains". This is maybe not going to come up much (I suspect people don't compile python directly into things much) but we should still fix it beacuse I don't want someone to copy this anti-pattern when building a new checker.