tox: Don't run tests agains ZODB+PR183 anymore

The patch that provides raw-extension functionality was merged into ZODB 5.6:

https://github.com/zopefoundation/ZODB/commit/2f8cc67a3ba3

So when testing with ZODB5 >= 5.6 the tests will excercise code path
that uses txn.extension_bytes, and when testing with ZODB4 the tests
will excercise code path that work-arounds lack of txn.extension_bytes.

/reviewed-by @jerome
/reviewed-on !18

tox.ini

 # zodbtools | tox setup
 [tox]
-envlist = py27-ZODB3, py{27,36,37}-ZODB{4,5,devPR183}
+envlist = py27-ZODB3, py{27,36,37}-ZODB{4,5}
 
 [testenv]
 deps	=
@@ -20,11 +20,7 @@ deps	=
     ZODB4:  transaction <2.0dev
 
     # latest current ZODB 5
-    ZODB5:  ZODB >=5.0,	<6.0dev
+    ZODB5:  ZODB >=5.6,	<6.0dev
     ZODB5:  ZEO	 >=5.0, <6.0dev
 
-    # with the patches from https://github.com/zopefoundation/ZODB/pull/183
-    ZODBdevPR183: -e git+https://github.com/navytux/ZODB@7f5a67b3e0d2dcf2d13f18a42a82e526f6d4055c#egg=ZODB
-    ZODBdevPR183: ZEO >=5.0, <6.0dev
-
 commands= {envpython} -m pytest