[MRG] Use pytest for asserting exceptions in all test methods.

[kdexd]

Fourth Phase PR on #411 (Succeeding PR #463 )

pytest.raises produces a less noisy error log upon failure. This PR replaces all the assert_raises and assert_raises_regex calls with pytest_assert_raises. Finally the unittest 's assert_raises is removed from joblib.testing and pytest_assert_raises has been renamed as raises.

• assert_raises and assert_raises_regex calls have been modified as:

with raises(ExceptionName) as excinfo:   # pytest.raises
    code_that_raises_exception()
    # more code
excinfo.match('exception message regex')

• assert_raises (old) and assert_raises_regex were the last of helpers used from unittest module. By this PR, the testing infrastructure no longer directly depends on unittest.

[lesteve]

I would use raises = pytest.raises it is not really asserting anything any more.

[kdexd]

Done in next commit. Although I am skeptical about the changing of old assert_raises calls. Either we should avoid this renaming or replce old assert_raises calls with with blocks as well...

[kdexd]

@lesteve for example, statements like this will seem weird. I did not use a with statement where regex is not checked. What do you think about this ?

[lesteve]

Using blocks (i.e. with with statements) is nicer indeed.

[lesteve]

LGTM, I just pushed a minor cosmetic change. I'll wait for the CIs to be green and I'll merge this one.

[lesteve]

All righty, all green, merging, thanks a lot!

[kdexd]

Great ! 😄 Time for warnings now..