Graph._demes_map attr.ib should use repr=False.

[grahamgower]

We don't want to see the _demes_map dict in the __repr__ string.

>>> import demes
>>> g=demes.load("examples/two_epoch.yml")
>>> from pprint import pprint
>>> pprint(repr(g))
("Graph(description='Single-population two-epoch demography.', "
 "time_units='generations', generation_time=None, doi=[], "
 "demes=[Deme(name='deme0', description='A deme that doubles in size 100 "
 "generations ago.', start_time=inf, ancestors=[], proportions=[], "
 'epochs=[Epoch(start_time=inf, end_time=100, start_size=1000, end_size=1000, '
 "size_function='constant', selfing_rate=0, cloning_rate=0), "
 'Epoch(start_time=100, end_time=0, start_size=2000, end_size=2000, '
 "size_function='constant', selfing_rate=0, cloning_rate=0)])], migrations=[], "
 "pulses=[], _deme_map={'deme0': Deme(name='deme0', description='A deme that "
 "doubles in size 100 generations ago.', start_time=inf, ancestors=[], "
 'proportions=[], epochs=[Epoch(start_time=inf, end_time=100, start_size=1000, '
 "end_size=1000, size_function='constant', selfing_rate=0, cloning_rate=0), "
 'Epoch(start_time=100, end_time=0, start_size=2000, end_size=2000, '
 "size_function='constant', selfing_rate=0, cloning_rate=0)])})")