Mutation of OrderedDict iterator during the loop

[eywalker]

In base_relation.py delete method, we modify the content of the OrderedDict relations while traversing using its values() iterator, thus modifying the iterator!

datajoint-python/datajoint/base_relation.py

Lines 360 to 367 in 25dff09

	for relation in relations.values():
	count = len(relation)
	if count:
	do_delete = True
	if config['safemode']:
	print(relation.full_table_name, '(%d tuples)' % count)
	else:
	relations.pop(relation.full_table_name)

It turns out that modification of the OrderedDict iterator is OK in Python 3.4 (although still not recommended) but simply fails in Python 3.5. The code has been fixed such that we are no longer using direct iterator if the OrderedDict is to be modified in the loop.

I also added Python 3.5 to the test target platform for Travis CI, addressing #198

[coveralls]

Coverage remained the same at 75.628% when pulling 7d1f66d on eywalker:master into 25dff09 on datajoint:master.