test compatibility with PyMongo 3.7

Author: dcrosta

tox.ini

@@ -2,7 +2,7 @@
 
 ; keep the pymongo list in sync with what's in .travis.yaml
 envlist=
-    pymongo{30,31,32,33,34,35,36}-mongo{32,34,36}-flask{0_11,0_12,10}, style
+    pymongo{30,31,32,33,34,35,36,37}-mongo{32,34,36}-flask{0_11,0_12,10}, style
 
 [testenv]
 docker =
@@ -20,6 +20,7 @@ deps =
     pymongo34: pymongo>=3.4,<3.5
     pymongo35: pymongo>=3.5,<3.6
     pymongo36: pymongo>=3.6,<3.7
+    pymongo36: pymongo>=3.7,<3.8
 
     flask0_11: flask>=0.11,<0.12
     flask0_12: flask>=0.12,<1.0