Note that we now raise ValueError from doom() if not in a doomable state,
rather than asserting.

Note that we now raise ValueError if passed a non-positive count, rather
than asserting.

Note that we now raise ValueError if asked to free a foreign TXN, rather
than asserting.

Add 'tox' testing for Python 3.3.

svn merge -r127104:127105 svn+ssh://svn.zope.org/repos/main/transaction/branches/chrism-fix.nonretryablecommitexceptions