Stop comparing repr()'s in TestEntryPoint

[s-t-e-v-e-n-k]

In Python 3, the default order of iterables can not determined,
so comparing the repr of objects that include tuples is not static
like it is under Python 2. Compare the attributes of EntryPoint
instead, making sure to sort .attrs and .extras.

Closes: #526

[jaraco]

Fantastic. Thanks!

One clarification - tuples and lists are ordered and will always have consistent order, but on late versions of Python, it's possible that ordering of items in identical sets is not stable, depending on the PYTHONHASHSEED:

$ PYTHONHASHSEED=3 python3.5 -c "print(list({'extra1', 'extra2'}) == list({'extra2', 'extra1'}))"
False
$ PYTHONHASHSEED=0 python3.5 -c "print(list({'extra1', 'extra2'}) == list({'extra2', 'extra1'}))"
True