Attempt to insulate against ZODB dependencies.

Mark (temporarily) tests which use DB, etc. as functests.

Conditionally import the BTreesConflictError exception type from
ZODB.POSError into BTrees.Interfaces;  if missing, derive from ValueError.

Note that this is basically what the C code does already, so make
it use the one from BTrees.Interfaces directly.

BTrees/BTreeModuleTemplate.c

@@ -485,7 +485,7 @@ INITMODULE (void)
 	return;
 
     /* Grab the ConflictError class */
-    m = PyImport_ImportModule("ZODB.POSException");
+    m = PyImport_ImportModule("BTrees.Interfaces");
     if (m != NULL) {
   	c = PyObject_GetAttrString(m, "BTreesConflictError");
   	if (c != NULL)

BTrees/Interfaces.py

@@ -507,6 +507,12 @@ class IIntegerFloatBTreeModule(IBTreeModule, IMerge):
     family = Attribute('The IBTreeFamily of this module')
 
 
+try:
+    from ZODB.POSException import BTreesConflictError
+except ImportError:
+    class BTreesConflictError(ValueError):
+        pass
+
 ###############################################################
 # IMPORTANT NOTE
 #