Add structured version info for dbm.gnu

[serhiy-storchaka]

Feature or enhancement

_gdbm._GDBM_VERSION (a private plain tuple from the GDBM_VERSION_MAJOR, GDBM_VERSION_MINOR and GDBM_VERSION_PATCH macros, added in gh-78082 for tests) is the only information about the gdbm library, and it only describes the version the module was built against. The version of the library loaded at runtime is not exposed, although gdbm provides it as gdbm_version_number[] and as the release string gdbm_version (a banner like 'GDBM version 1.26. 30/07/2025', possibly with a build date).

Add dbm.gnu.GDBM_VERSION_INFO for the build-time version, dbm.gnu.gdbm_version_info for the runtime version (both named tuples), and dbm.gnu.gdbm_version for the release string.

Linked PRs

• gh-157478: Add structured version info for dbm.gnu #157479