fix(deps): remove setuptools from dependencies (#339)

* fix(deps): remove setuptools from dependencies

Fixes #338

* chore: remove setuptools from constraints
diff --git a/setup.py b/setup.py
index ddc5600..e400a52 100644
--- a/setup.py
+++ b/setup.py
@@ -33,7 +33,6 @@
     "protobuf >= 3.12.0",
     "google-auth >= 1.25.0, < 3.0dev",
     "requests >= 2.18.0, < 3.0.0dev",
-    "setuptools >= 40.3.0",
 ]
 extras = {
     "grpc": ["grpcio >= 1.33.2, < 2.0dev", "grpcio-status >= 1.33.2, < 2.0dev"],
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 0c2a07b..bb1eb1e 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -9,7 +9,6 @@
 protobuf==3.12.0
 google-auth==1.25.0
 requests==2.18.0
-setuptools==40.3.0
 packaging==14.3
 grpcio==1.33.2
 grpcio-gcp==0.2.2