C++14 or C++17 support

It would be nice to support C++14 (or newer versions) in python extensions.

The older python images in `.circleci/config.yml` appear to be for Debian jesse, which has GCC 4. Jesse is EOL since June, so I think these images could at least be replaced by images on stretch, which has GCC 6. Buster comes with GCC 8. I believe all the relevant versions of Clang and MSVC have C++17 support. GCC 6 supports C++14 and parts of C++17, and GCC 8 has essentially complete support of C++17. Also `setup.py` explicitly states `--std=c++11` for compiler and linker flags on osx and linux. This could be changed to `c++14` or `c++17` or left off to use the compiler default (C++14 for current GCC and Clang).