Python: Only install pdoc for Python 3.9+

It's used to generate the docs, which we do on Python 3.10 or later on CI

glean-core/python/requirements_dev.txt

@@ -2,7 +2,7 @@ auditwheel==6.0.0
 coverage[toml]==7.2.2
 jsonschema==3.2.0
 mypy==1.4.1
-pdoc3==0.11.5
+pdoc3==0.11.5; python_version > '3.8'
 pip
 pytest-localserver==0.8.0
 MarkupSafe==2.0.1