[ibm-db] Mark constants as Final

[brianschubert]

Also

• Add missing __version__
• Set literal values for a few constants. These all have hard-coded values and have been stable since being introduced. I sanity-checked the values using stubtest from mypy's master branch, which now checks Final values (python/mypy#20858)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉