Fix exception assertions' content

[macisamuele]

The goal of this PR is to fix the failing tests.

[coveralls]

Coverage remained the same at 99.574% when pulling eff92af on macisamuele:maci-fix-tests into c13bcd3 on striglia:master.

[striglia]

Looks good thanks @macisamuele. I'm going to see why I didn't get any emails from Travis about this...

(happy to ship+merge if you like, or we can have Stephan do it too 👍)

[sjaensch]

I'm wondering why these tests are passing on appveyor...

[macisamuele]

@sjaensch For what I see travis executed daily, while appveyor is executed only in case of changes.
Checking the first time pyramid-swagger failed to run tests on master (travis) I see that they started to fail once pytest>=5 was released.

Checking pytest changelog I see

#5412: ExceptionInfo objects (returned by pytest.raises) now have the same str representation as repr, which avoids some confusion when users use print(e) to inspect the object.

This means code like:

with pytest.raises(SomeException) as e:
    ...
assert "some message" in str(e)

Needs to be changed to:

with pytest.raises(SomeException) as e:
    ...
assert "some message" in str(e.value)

which is exactly the content of this PR