Test coverage for ZODB{3,4,5}

Use tox to test with all kinds of ZODB.
With preceding 3 patches tests pass with all versions of upstream ZODB.

TODO: test coverage for both py2 and py3.

.gitignore

 /dist
 /zodbtools.egg-info
+
+/.tox/

tox.ini

+# zodbtools | tox setup
+[tox]
+envlist = py27-{ZODB3,ZODB4,ZODB5}
+
+[testenv]
+deps	=
+    # XXX tox does not consult extras_require['test'] -> install pytest explicitly
+    pytest
+
+    # latest ZODB from ZODB3 series
+    ZODB3:  ZODB3 >=3.10, <3.11dev
+    ZODB3:  transaction <2.0dev
+    # ZConfig 3.2.0 passes filename to ZEO config as unicode which eventualy breaks in FileStorage:
+    # https://github.com/zopefoundation/ZODB/blob/3.10.7-4-gb8d7a8567/src/ZODB/FileStorage/FileStorage.py#L1640
+    ZODB3:  ZConfig <3.2.0
+
+    # latest current ZODB 4
+    ZODB4:  ZODB >=4.0, <5.0dev
+    ZODB4:  ZEO  >=4.0, <5.0dev
+    # ZEO4 depends on transaction <2
+    ZODB4:  transaction <2.0dev
+
+    # latest current ZODB 5
+    ZODB5:  ZODB >=5.0,	<6.0dev
+    ZODB5:  ZEO	 >=5.0, <6.0dev
+
+
+commands= {envpython} -m pytest