Fix generating docs in readthedocs

as we (they?) are still using Python 2 there

docs/conf.py

@@ -21,7 +21,7 @@
 master_doc = "index"
 
 project = "pypuppetdb"
-copyright = f"2013-{datetime.now().year}, Vox Pupuli"
+copyright = "2013-%s, Vox Pupuli" % datetime.now().year
 version = pkg_resources.get_distribution(project).version
 release = version