Only build documentation with Python 3.9 or newer

The documentation requires pydata-sphinx-theme 0.15, which is not
available for Python 3.8.

noxfile.py

@@ -2,6 +2,7 @@ import nox
 import os
 from pathlib import Path
 import shutil
+import sys
 
 
 # Ensure that nox supports session tags.
@@ -76,6 +77,9 @@ def tests(session):
 @nox.session(reuse_venv=True)
 def docs(session):
     """Build the HTML documentation."""
+    if sys.version_info.major == 3 and sys.version_info.minor < 9:
+        # We need Python 3.9 in order to install pydata-sphinx-theme 0.15.
+        session.skip('We need Python 3.9 or later to build docs')
     session.install('-r', 'requirements-rtd.txt')
     session.run(
         'sphinx-build', '-W', '-b', 'html', './doc', './doc/build/html'