chore: upgrade sphinx to 4.0.1 (#256)

* ci: use the latest version of sphinx

* 🦉 Updates from OwlBot

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

noxfile.py

@@ -192,7 +192,7 @@ def docs(session):
     """Build the docs for this library."""
 
     session.install("-e", ".")
-    session.install("sphinx<3.0.0", "alabaster", "recommonmark")
+    session.install("sphinx==4.0.1", "alabaster", "recommonmark")
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
     session.run(
@@ -215,7 +215,7 @@ def docfx(session):
 
     session.install("-e", ".")
     session.install(
-        "sphinx<3.0.0", "alabaster", "recommonmark", "gcp-sphinx-docfx-yaml"
+        "sphinx==4.0.1", "alabaster", "recommonmark", "gcp-sphinx-docfx-yaml"
     )
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)

owlbot.py

@@ -119,7 +119,4 @@
 # ----------------------------------------------------------------------------
 python.py_samples(skip_readmes=True)
 
-# TODO(busunkim): Use latest sphinx after microgenerator transition
-s.replace("noxfile.py", """['"]sphinx==4.0.1['"]""", '"sphinx<3.0.0"')
-
 s.shell.run(["nox", "-s", "blacken"], hide_output=False)