bpo-1635741: Port _dbm module to multiphase initialization

[corona10]

https://bugs.python.org/issue1635741

[corona10]

@shihai1991 PTAL!

[corona10]

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

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

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

        import test.support
        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]

@shihai1991 Thanks shihai! I've updated it

[shihai1991]

Thanks, LGTM.

[bedevere-bot]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[corona10]

I have made the requested changes; please review again

[vstinner]

Thanks! It's now much better!

[vstinner]

LGTM, thanks!