more explicit test
improve help and move to argparse
improve help and move to argparse
first working version of a __len__ cache.

This shouldd be improved to make sure that len(bt) is always fast.
second version, quite efficient. It is about 20% slower than ignoringg len().
The advantage is that this does not block a GUI.
use zodbpickle to resolve pickling problems when using a durus db from python2
Merge branch 'hotfix/zodbpickle'
Merge branch 'hotfix/zodbpickle' into develop
make persistent.py compatible with _persistent.so
Merge branch 'hotfix/_persistent.so_compatible'
Merge branch 'hotfix/_persistent.so_compatible' into develop
Merge branch 'develop' into feature/len_support

* develop:
  make persistent.py compatible with _persistent.so
  use zodbpickle to resolve pickling problems when using a durus db from python2
Merge branch 'master' into feature/len_support

* master:
Merge branch 'feature/len_support' into develop
consolidate changes (merge problem)
replace tab by 4 spaces
finished len support with an explicit calculation.
Bumped the version to 3.9.3 .
Because the len calculation made most _p_notify calls avoidable, the time penalty
is only 6 percent, and I am going to always enable len support.
This is still compatible because the old recursive function for len calculation is still there.
adjust text for upload