fix(python_library): fix external unit test dependencies

I recently submitted https://github.com/googleapis/synthtool/pull/811/files, allowing external dependencies for unit tests. This fixes a small missing comma bug

Source-Author: Daniel Sanche <d.sanche14@gmail.com>
Source-Date: Thu Oct 29 16:58:01 2020 -0700
Source-Repo: googleapis/synthtool
Source-Sha: 6542bd723403513626f61642fc02ddca528409aa
Source-Link: googleapis/synthtool@6542bd7

noxfile.py

@@ -72,7 +72,9 @@ def default(session):
     # Install all test dependencies, then install this package in-place.
     session.install("asyncmock", "pytest-asyncio")
 
-    session.install("mock", "pytest", "pytest-cov")
+    session.install(
+        "mock", "pytest", "pytest-cov",
+    )
     session.install("-e", ".")
 
     # Run py.test against the unit tests.

synth.metadata

@@ -19,14 +19,14 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ea52b8a0bd560f72f376efcf45197fb7c8869120"
+        "sha": "6542bd723403513626f61642fc02ddca528409aa"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ea52b8a0bd560f72f376efcf45197fb7c8869120"
+        "sha": "6542bd723403513626f61642fc02ddca528409aa"
       }
     }
   ],