python: bulk port tdb iterkeys for py3

python: bulk port tdb iterkeys for py3

In py3, `dict.iterkeys()` is removed, we need to use `keys()` instead.
This is compatible with py2 since `dict.keys()` exists for py2.

tdb pretents to be a dict, however, not completely.
It provides `iterkeys()` for py2 only, and `keys()` for py3 only,
which means replace `iterkeys()` to `keys()` will break py2.

In python, iter a dict will implicitly iter on keys.
Use this feature to work around.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>