Resolve str AssertionErrors on ExceptionInfo objects in pytest 5

[dhomeier]

Attempt to fix errors of the kind

>       assert "A Latitude angle cannot be created from a Longitude angle" in str(excinfo)
E       AssertionError: assert 'A Latitude angle cannot be created from a Longitude angle' in '<ExceptionInfo TypeError tblen=2>'
E        +  where '<ExceptionInfo TypeError tblen=2>' = str(<ExceptionInfo TypeError tblen=2>)

that started to appear with pytest 5.0.0 as discussed in #8934.
General remedy is to call str(excinfo.value) rather than str(excinfo), but there are a few cases where excinfo has no method value - see in particular patches to io and table. Apparently some hand-crafted or derived exceptions like FileNotFoundError or fits.VerifyError still return a plain string instead of an ExceptionInfo, but I could not find any hints in the pytest docs how to properly declare exception classes to better handle this.
Also excinfo.value does not contain the exception name, but since this is generally checked against immediately before, I think it can be safely omitted from the string comparison.
On the plus side, ExceptionInfo provides its own match method, simplifying some re comparisons (cf. test_html.py and test_wcsprm.py).
Tested with pytest 5.0.0 on Python 3.5 and 3.8; 4 unrelated visualisation.wcsaxes failures that turn up identically with pytest 3.10.1.

[dhomeier]

Would the changelog entry go under Bug fixes: astropy.tests?

[bsipocz]

@dhomeier - no, this is neither user facing nor has any real behaviour change, so don't need a changelog.

[bsipocz]

Thanks @dhomeier!

[dhomeier]

@bsipocz - no, definitely no user-relevant changes here, but it might be useful to document for casual developers and contributors - don't know where to best put it though.