Wrong version of cmake in stretch-backports

[Ondrej Novy <novy@ondrej.org>]

Hi,

cmake versions:

stretch: cmake (3.7.2-1)

stretch-bpo: cmake (3.16.3-1~bpo9+1)

buster: cmake (3.13.4-1)

So it looks like we have newer cmake in stretch-bpo than in buster, which is wrong. See #954852

We should probably remove cmake (3.16.3-1~bpo9+1) or move it to stretch-bpo-sloopy.

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Best regards
 Ondřej Nový