bpo-1635741: Port _gdbm module to multiphase initialization

[corona10]

https://bugs.python.org/issue1635741

[corona10]

I' ve checked the memory leak with these scripts and no leaks found

./python.exe -m test test_dbm_gnu -R 3:3

def test_gdbm(self):
    code = textwrap.dedent(r"""
        import glob

        import test.support
        dbm = test.support.import_module('_gdbm')
        # or dbm = test.support.import_module('dbm')
        _fname = test.support.TESTFN

        def delete_files():
            for f in glob.glob(_fname + "*"):
                test.support.unlink(f)

        f = dbm.open(_fname, 'n')
        f[b'g'] = b"indented"
        f.close()
        delete_files()
    """)
    ret = test.support.run_in_subinterp(code)
    self.assertEqual(ret, 0)

[corona10]

Note that dbm = test.support.import_module('dbm') is leaked at the master branch :)

[vstinner]

LGTM.

Minor remarks:

• I'm not sure why only a few methods uses the defining class, whereas some others use Py_TYPE(self).
• nitpick: some "clinic input" uses an empty line betwen the function name and its parameters, sometimes there is none.

But I don't think that it's worth it to address these minor things. If you care, you may also update _dbmmodule.c. Maybe in a follow-up PRs.

[vstinner]

Note that dbm = test.support.import_module('dbm') is leaked at the master branch :)

Maybe open an issue to track this bug.

[vstinner]

Maybe open an issue to track this bug.

Well, I added a comment to: https://bugs.python.org/issue40987#msg371679

[corona10]

@vstinner

Maybe open an issue to track this bug

Oh I mean the subinterpreter test with dbm = test.support.import_module('dbm')is leaked
With this change, there is no leak.

[corona10]

@vstinner Thanks for the review!

[vstinner]

With this change, there is no leak.

Oh ok, that's great! Sorry, I misunderstood your comment.