debian: Use Python 3 instead of 3.4
debian: Use Python 3 instead of 3.4

There's no reason to force an explicit minor version.

Our main target, Debian Jessie, has 3.4 anyway, and the python3 symlink
is pointing to it.

If we ever move to Stretch, which has 3.5, then this will keep working
the same way.

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