/usr/share/binfmts/python3.5 is in python3.5-minimal 3.5.3-1+deb9u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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package python3.5 interpreter /usr/bin/python3.5 magic \x17\x0d\x0d\x0a