Make bin/ask_update.py grab the right sympy
New assumptions: zero is even

This also lets it deduce that zero is an integer.
Use a context manager in ask_update.py
Sort the keys in the ask_generated.py dictionary

This will make the diffs much less drastic when new facts are added.
Always generate ask_generated.py with set([]) notation

In Python 3, repr(set) uses {} notation, which is not compatible with Python
2.6.

This also sorts the entries of the sets, so that running ask_update.py without
any changes should always result in the exact same ask_generated.py.
Merge pull request #2513 from asmeurer/zero_is_even

Add zero is even fact to new assumptions