This greatly improves startup time as pyinstaller doesn't have to
unpack everything on startup. The same also applies to macOS and Linux,
but there we 1) don't have installers to hide all the files and
2) have a filesystem that deals much better with lots of small files.
Additionally, simplify cibuild to be a bit more reasonable.
Cater for the corner case where commits are incorporated on a maintenance
branch. We should be able to test these without adding a dev suffix to the tool
versions.
Whenever TRAVIS_TAG is non-empty, TRAVIS_BRANCH is over-ridden to match the
TRAVIS_TAG value. Adjust our tests to reflect this, and add a sanity check that
fails hard if this constraint is ever not met.
See https://github.com/travis-ci/travis-ci/issues/4745
