tox/ZODB3: Limit transaction < 2.0dev

In 2.0 transaction made an incomatible change to require description to
always be unicode, not string. ZODB3 was not updated.

Reference: https://github.com/zopefoundation/transaction/pull/32

tox.ini

@@ -11,6 +11,7 @@ deps    =
 
     # latest ZODB from 3 series
     ZODB3:  ZODB3 >=3.10, <3.11dev
+    ZODB3:  transaction <2.0dev
 
     # latest current ZODB _4_
     ZODB4:  ZODB >=4.0, <5.0dev