Remove Python 3.4 builds from AppVeyor configuration

[pont-us]

Python 3.4 builds on Mac have broken, evidently due once again to problems with installation of the long-unsupported OpenSSL package. There have been no objections to removing Python 3.4 support in the next SNAP release, so this would be a good moment to finally drop Python 3.4 for all platforms, shrink the support matrix, and simplify the build process.