chore: update noxfile (via synth) (#10)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2020-04-21 06:13:29,482 synthtool > Executing /tmpfs/src/git/autosynth/working_repo/synth.py.
On branch autosynth
nothing to commit, working tree clean
.coveragerc
.flake8
.github/CONTRIBUTING.md
.github/ISSUE_TEMPLATE/bug_report.md
.github/ISSUE_TEMPLATE/feature_request.md
.github/ISSUE_TEMPLATE/support_request.md
.github/PULL_REQUEST_TEMPLATE.md
.github/release-please.yml
.gitignore
.kokoro/build.sh
.kokoro/continuous/common.cfg
.kokoro/continuous/continuous.cfg
.kokoro/docs/common.cfg
.kokoro/docs/docs.cfg
.kokoro/presubmit/common.cfg
.kokoro/presubmit/presubmit.cfg
.kokoro/publish-docs.sh
.kokoro/release.sh
.kokoro/release/common.cfg
.kokoro/release/release.cfg
.kokoro/trampoline.sh
CODE_OF_CONDUCT.md
CONTRIBUTING.rst
LICENSE
MANIFEST.in
docs/_static/custom.css
docs/_templates/layout.html
docs/conf.py.j2
noxfile.py.j2
renovate.json
setup.cfg
Running session blacken
Creating virtual environment (virtualenv) using python3.6 in .nox/blacken
pip install black==19.3b0
black docs google tests noxfile.py setup.py
reformatted /tmpfs/src/git/autosynth/working_repo/docs/conf.py
reformatted /tmpfs/src/git/autosynth/working_repo/noxfile.py
All done! ✨ 🍰 ✨
2 files reformatted, 16 files left unchanged.
Session blacken was successful.
2020-04-21 06:13:33,273 synthtool > Wrote metadata to synth.metadata.

```
</details>

Author: yoshi-automation

packages/google-cloud-dns/noxfile.py

@@ -110,8 +110,7 @@ def system(session):
 
     # Install all test dependencies, then install this package into the
     # virtualenv's dist-packages.
-    session.install("mock", "pytest")
-
+    session.install("mock", "pytest", "google-cloud-testutils")
     session.install("-e", ".")
 
     # Run py.test against the system tests.
@@ -139,7 +138,7 @@ def docs(session):
     """Build the docs for this library."""
 
     session.install("-e", ".")
-    session.install("sphinx", "alabaster", "recommonmark")
+    session.install("sphinx<3.0.0", "alabaster", "recommonmark")
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
     session.run(

packages/google-cloud-dns/synth.metadata

@@ -1,11 +1,17 @@
 {
-  "updateTime": "2020-03-31T12:13:26.210653Z",
   "sources": [
+    {
+      "git": {
+        "name": ".",
+        "remote": "git@github.com:googleapis/python-dns",
+        "sha": "5bd85d2b856ed29da9b6a975673562ae40eb7283"
+      }
+    },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "a003d8655d3ebec2bbbd5fc3898e91e152265c67"
+        "sha": "cdddf139b36000b3a7c65fd2a7781e253262359a"
       }
     }
   ]