TST fix tests that became broken after adding top-level imports and s…

…witching to py.test.

py.test replaces AssertionError exception with its own subclass in an import hook.
For some reason when top-level shortcuts were added "except AssertionError" in
contracts.__init__ meant "except py.test AssertionError" while scrapy.exceptions.ContractFail
was still a subclass of a builtin AssertionError, so Contracts reported errors
instead of failures and this made tests in test_contracts.py fail.

For now - just disable fancy py.test asserts.

pytest.ini

@@ -2,4 +2,4 @@
 [pytest]
 usefixtures = chdir setlog
 python_files=test_*.py __init__.py
-addopts = --doctest-modules
+addopts = --doctest-modules --assert=plain