Import Iterable from collections.abc for Py 3.9 compatibility (#58)

Import Iterable from collections.abc instead of collections for Python 3.9 compatibility, and add Python 3.9 to CI.

Author: tirkarthi

.travis.yml

@@ -6,6 +6,7 @@ python:
   - 3.6
   - 3.7
   - 3.8
+  - 3.9-dev
   - pypy
   - pypy3
 cache: pip

gql/dsl.py

@@ -1,4 +1,3 @@
-import collections
 import decimal
 from functools import partial
 
@@ -10,6 +9,11 @@
 
 from .utils import to_camel_case
 
+if six.PY3:
+    from collections.abc import Iterable
+else:
+    from collections import Iterable
+
 
 class DSLSchema(object):
     def __init__(self, client):
@@ -134,7 +138,7 @@ def query(*fields):
 
 
 def serialize_list(serializer, values):
-    assert isinstance(values, collections.Iterable), 'Expected iterable, received "{}"'.format(repr(values))
+    assert isinstance(values, Iterable), 'Expected iterable, received "{}"'.format(repr(values))
     return [serializer(v) for v in values]