* don't touch the website for prereleases. Ideally, we should generate a different download and documentation page for each release, including prereleases, but only change the "main"/unversioned pages for stable releases
* made "announce" re-read the changes from file, so that it is more independent of the rest of the release process
* check for invalid prerelease tags earlier
* run tests on the executables instead of the bundles (bundles are now only tested for integrity)
   this means we have the opportunity to fix any issue found by the test before updating/committing the changelog
* website and changes.rst commits are only pushed at the very end of the release process so that I can do most of it without internet
* allow to use a local repository as the source
* restore cwd at the end of the script
blank line for cleaner log
fix announce