Fixed types

[davidbrochart]

Changes

Fixes #134.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #123, the entry should look like this:

- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/asphalt-framework/asphalt/issues/123>`_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

[coveralls]

coverage: 97.527%. remained the same
when pulling 80f6e6e on davidbrochart:fix-types
into c7f994a on asphalt-framework:5.0.

[agronholm]

Two issues I'd like to point out:

1. There is no accompanying regression test
2. How much sense does the is_class() test make after this change? Wouldn't it be better to treat the case of a Sequence as special, and default to assuming that types is a class?