Also add a compile time check for SIZEOF_VOID_P.

This fixes #2670.

See also
https://bugs.python.org/issue35037
https://bugs.python.org/issue4709

gh2638 absolute import handling

This change was actually unnesarry as with the new regex we iterate
over all possible combinations of package and modules.

Fix: absolute import handling

Do not install test dependencies on CPython dev versions in travis as they are likely to fail in pre-releases.