Fixes a failing test in astm/tests/test_mapping.py (test_proxy_slices).

At least as of Python 3.6, slice operations end up calling __getitem__
and __setitem__ with a slice object instead of the deprecated
__getslice__ and __setslice__.

The server's Order model does not use that field (but the client does).

I assume the test is wrong rather than the model, but if not, then the
field could be added to the model instead.  Not clear to me from VCS
history what the intent was, though.

Fixes the failing test.

Mostly unrecognized escape sequences because of bare backslashes.

Tests are now free of warnings on 3.6.  There may be other warnings
produced by the library if the code isn't covered by tests, but those
can be fixed as they're uncovered.