This was already observed with version 0.12.7, see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=753220 which was closed because unreproducible.
The problem is also known to upstream (https://github.com/bottlepy/bottle/issues/618, https://github.com/bottlepy/bottle/issues/891), the author of the package states that "the server tests are unstable (and should probably be disabled in debian builds because they are more integration than unit tests)" One way of patching it would be to pass the "fast" option ("Skip server adapter tests") to the testall.py script in the Makefile, line 33.
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