chore: migrate to owl bot (#87)

This PR migrates from autosynth to [owl bot](https://github.com/googleapis/repo-automation-bots/tree/master/packages/owl-bot).  owl bot will save time for maintainers as it will automatically open PRs when there are updates in [googleapis-gen](https://github.com/googleapis/googleapis-gen/tree/master/google) without requiring maintainers to run `synthtool` to build the client from protos. Additionally, similar to autosynth, PRs will be automatically opened when there are template updates. With owl bot, on every PR, a post-processor image will run so that we won't have to ask contributors to run [blacken](https://github.com/googleapis/python-talent/blob/master/synth.py#L65).

talent/noxfile.py

@@ -172,10 +172,16 @@ def blacken(session: nox.sessions.Session) -> None:
 def _session_tests(session: nox.sessions.Session, post_install: Callable = None) -> None:
     """Runs py.test for a particular project."""
     if os.path.exists("requirements.txt"):
-        session.install("-r", "requirements.txt")
+        if os.path.exists("constraints.txt"):
+            session.install("-r", "requirements.txt", "-c", "constraints.txt")
+        else:
+            session.install("-r", "requirements.txt")
 
     if os.path.exists("requirements-test.txt"):
-        session.install("-r", "requirements-test.txt")
+        if os.path.exists("constraints-test.txt"):
+            session.install("-r", "requirements-test.txt", "-c", "constraints-test.txt")
+        else:
+            session.install("-r", "requirements-test.txt")
 
     if INSTALL_LIBRARY_FROM_SOURCE:
         session.install("-e", _get_repo_root())