refactor(tests): fix few tests, use pytest.raises, extend raises helper

[antoni-szych-rtbhouse]

Hi, I thought it would be nice to modernize tests a bit:

• use with pytest.raises() or with raises() in multiple places
• parametrize a few tests
• make raises() manager throw AssertionError if expected exception was not raised inside the context
• fix 1 or 2 asserts in tests that were incorrectly working due to the above
• remove python 2 or <3.2 compatibility code (note: this extends beyond tests)

I have some doubts regarding existing tests if they really test what they say they do, especially test_self_any and test_self_all. However it should be checked by someone with more experience (I haven't used Self-related validation yet myself)

[alecthomas]

Good idea!