gh-134004: Dbm vacuuming

[Andrea-Oliveri]

gh-134004:

1. Added a .reorganize() method to dbm.sqlite and dbm.dumb which allows to retrieve unused space from database files. Name was chosen for consistency with dbm.gnu.reorganize(). dbm.ndbm` does not support any such optimization.
2. dbm.dumb.reorganize() is a new custom implementation rewriting the database file in-place and removing the gaps left by deletions, then truncating the excess.
3. Added 3 tests to ensure .reorganize() runs as expected on all compatible dbm submodules.
4. Updated the documentation of dbm module to include contributed methods as well as to highlight some of the limitations of the current implementations, such as lack of concurrency compatibility (for dbm.dumb) and lack of automatic reorganizing
5. Added .reorganize() method to shelve to expose dbm's submodules methods
6. Added documentation for shelve.reorganize()

Note: Unfortunately I did not feel comfortable changing the C code at this time, so I chose not to add an empty .reorganize() method to dbm.ndbm. This resulted in:

1. Inconsistencies between the methods supported by dbm submodules.
2. The added tests need to verify, using hasattr(), whether the dbm objects have the .reorganize() method and succeed early if that is not the case.
3. The documentation clearly explains the situation described here.

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