pybind/mgr/snap_schedule: datetime.fromisoformat fallbacks

pybind/mgr/snap_schedule: datetime.fromisoformat fallbacks

fromisoformat was introduced in pytho3.7. This can be worked around by
either installing
https://github.com/movermeyer/backports.datetime_fromisoformat. If a
pre3.7 python is found and the backport module is not installed,
snap_schedule falls back to parsing a fully specified timestamp
format. This commit adds the necessary code.

Signed-off-by: Jan Fajerski <jfajerski@suse.com>