Modernize code to Python 3.6+ (#1158)

* Compact dict and set building
* Remove redundant parentheses
* Simplify chained conditions
* Change method name to lowercase
* Use triple double quotes for docstrings

Thanks, @eumiro!

datasette/facets.py

@@ -162,10 +162,8 @@ class ColumnFacet(Facet):
                 )
                 num_distinct_values = len(distinct_values)
                 if (
-                    num_distinct_values
-                    and num_distinct_values > 1
+                    1 < num_distinct_values < row_count
                     and num_distinct_values <= facet_size
-                    and num_distinct_values < row_count
                     # And at least one has n > 1
                     and any(r["n"] > 1 for r in distinct_values)
                 ):