Update gcloud.py for current Python versions (#2099)

buildkite/gcloud.py

diff --git a/buildkite/gcloud.py b/buildkite/gcloud.py
index 2407e31..6d8b585 100755
--- a/buildkite/gcloud.py
+++ b/buildkite/gcloud.py
@@ -31,7 +31,7 @@
 
 
 def is_sequence(seq):
-    return isinstance(seq, collections.Sequence) and not isinstance(seq, str)
+    return isinstance(seq, collections.abc.Sequence) and not isinstance(seq, str)
 
 
 def gcloud(*args, **kwargs):