fix(python): install testutils from pypi (#503)

https://pypi.org/project/google-cloud-testutils/

Author: busunkim96

synthtool/gcp/templates/python_library/noxfile.py.j2

@@ -117,8 +117,7 @@ def system(session):
 
     # Install all test dependencies, then install this package into the
     # virtualenv's dist-packages.
-    session.install("mock", "pytest", {% for d in system_test_external_dependencies %}"{{d}}"{% if not loop.last %},{% endif %}{% endfor %})
-    session.install("git+https://github.com/googleapis/python-test-utils")
+    session.install("mock", "pytest", "google-cloud-testutils", {% for d in system_test_external_dependencies %}"{{d}}"{% if not loop.last %},{% endif %}{% endfor %})
 
     {%- if system_test_local_dependencies %}
     session.install("-e", {% for d in system_test_local_dependencies %}"{{d}}"{% if not loop.last %},{% endif %}{% endfor %})