Auto-generate next version number based on nature of included changes

[ssbarnea]

To prevent human errors of like doing a minor version increase when changes includes changes marked as major, we should auto generate the next version number when creating draft body.

• we should update only the title, allowing release-manager to copy the value from title to tag when he does the release using https://github.com/pycontribs/tender/releases page.
• if a draft already exists and has a title assigned, this should be kept.
• generated version number should not be lower than the last pre-release tag found. If last tag was "1.2.0a0", the generated version is supposed to be "1.2.0".