Pysubnettree

* Update cmake submodule for changes related to Python 2 EOL (Jon Siwek, Corelight)

* Update CI: test newer Python versions and drop older versions (Jon Siwek, Corelight)

* Update Python invocations to use `python3` explicitly (Jon Siwek, Corelight)

* Change CMakeLists.txt to enforce Python >= 3.5 (Jon Siwek, Corelight)

* Update documentation for Python 2 EOL and new Python 3 requirements (Jon Siwek, Corelight)

* Fix use of CMake VERSION_GREATER_EQUAL (Jon Siwek, Corelight)

* Set CMake policies according to CMake version (Jon Siwek, Corelight)

CMake versions earlier than when the policy was introduced otherwise
emit an "unknown policy" error.

* Update CMake script for new SWIG policies. (Jon Siwek, Corelight)

* Fix btest.cfg's PYTHONPATH. (Jon Siwek, Corelight)

* Move header files into "include/ to avoid confusion on
case-insensitive file systems. (Jon Siwek, Corelight)

* Drop use of Python 2.6 for Travis CI tests (Jon Siwek, Corelight)

* Improve parsing/handling of invalid subnet strings (Shai Bentov)

* GH-12: fix segfault on invalid prefixes (Jon Siwek, Corelight)

* Fix python3 compatibility of ./configure script (Jon Siwek, Corelight)