Apply black to everything, enforce via unit tests (#449)

I've run the black code formatting tool against everything:

    black tests datasette setup.py

I also added a new unit test, in tests/test_black.py, which will fail if the code does not
conform to black's exacting standards.

This unit test only runs on Python 3.6 or higher, because black itself doesn't run on 3.5.

datasette/inspect.py

@@ -7,7 +7,7 @@ from .utils import (
     escape_sqlite,
     get_all_foreign_keys,
     table_columns,
-    sqlite3
+    sqlite3,
 )
 
 
@@ -29,7 +29,9 @@ def inspect_hash(path):
 
 def inspect_views(conn):
     " List views in a database. "
-    return [v[0] for v in conn.execute('select name from sqlite_master where type = "view"')]
+    return [
+        v[0] for v in conn.execute('select name from sqlite_master where type = "view"')
+    ]
 
 
 def inspect_tables(conn, database_metadata):
@@ -37,15 +39,11 @@ def inspect_tables(conn, database_metadata):
     tables = {}
     table_names = [
         r["name"]
-        for r in conn.execute(
-            'select * from sqlite_master where type="table"'
-        )
+        for r in conn.execute('select * from sqlite_master where type="table"')
     ]
 
     for table in table_names:
-        table_metadata = database_metadata.get("tables", {}).get(
-            table, {}
-        )
+        table_metadata = database_metadata.get("tables", {}).get(table, {})
 
         try:
             count = conn.execute(