Explicitly encode keys going into the URL cache db as UTF-8 just to be consistent
Fix a few examples of astropy.tests.helper.raises mistakenly being used like py.test raises (the latter works a context manager, the former previously did not).  But this also goes ahead an makes astropy.tests.helper.raises pass through to pytest.raises when used as a context manager.  A little ambiguous, but at the same time makes this mistake less likely.