doctests: fix problem with decorators

supersedes #7399

with this commit all decorated functions/methods containing doctests,
where the used decorator is != ``property`` are executed now and not
silently skipped.
runtests.py: it is safe to remove 'except ValueError' in finder

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
Merge pull request #7402 from thisch/doctestdecofix

doctests: fix problem with decorators