default log levels: DEBUG for dev, INFO for stable

[sebix]

set the default log level (internal fallback value) to

• DEBUG if the version number indicates this is a preliminary release (alpha, beta, dev)
• INFO if the version number indicates a stable release (last part is integer, release candidate or anything else

removes the need to check and adappt this manually at every release or starts of development cycles

Please don't merge before 3.3.1 release is done.