Fixed version of pymongo.

requirements.txt

@@ -44,6 +44,7 @@ elasticsearch==6.4.0
 msgpack<0.6.0
 celery[redis]
 mongoengine==0.19.1
+pymongo==3.10.1
 Werkzeug==0.16.1
 flask
 flask-restplus
@@ -88,7 +89,7 @@ pylint_mongoengine==0.3.3
 pycodestyle
 pytest==3.10.0  # celery fixtures not compatible with 4.x
 pytest-timeout
-pytest-cov
+pytest-cov==2.7.1
 rope
 mongomock==3.17.0
 names