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.

tests/test_csv.py

@@ -1,22 +1,26 @@
-from .fixtures import ( # noqa
+from .fixtures import (  # noqa
     app_client,
     app_client_csv_max_mb_one,
-    app_client_with_cors
+    app_client_with_cors,
 )
 
-EXPECTED_TABLE_CSV = '''id,content
+EXPECTED_TABLE_CSV = """id,content
 1,hello
 2,world
 3,
 4,RENDER_CELL_DEMO
-'''.replace('\n', '\r\n')
+""".replace(
+    "\n", "\r\n"
+)
 
-EXPECTED_CUSTOM_CSV = '''content
+EXPECTED_CUSTOM_CSV = """content
 hello
 world
-'''.replace('\n', '\r\n')
+""".replace(
+    "\n", "\r\n"
+)
 
-EXPECTED_TABLE_WITH_LABELS_CSV = '''
+EXPECTED_TABLE_WITH_LABELS_CSV = """
 pk,planet_int,on_earth,state,city_id,city_id_label,neighborhood,tags
 1,1,1,CA,1,San Francisco,Mission,"[""tag1"", ""tag2""]"
 2,1,1,CA,1,San Francisco,Dogpatch,"[""tag1"", ""tag3""]"
@@ -33,45 +37,47 @@ pk,planet_int,on_earth,state,city_id,city_id_label,neighborhood,tags
 13,1,1,MI,3,Detroit,Corktown,[]
 14,1,1,MI,3,Detroit,Mexicantown,[]
 15,2,0,MC,4,Memnonia,Arcadia Planitia,[]
-'''.lstrip().replace('\n', '\r\n')
+""".lstrip().replace(
+    "\n", "\r\n"
+)
 
 
 def test_table_csv(app_client):
-    response = app_client.get('/fixtures/simple_primary_key.csv')
+    response = app_client.get("/fixtures/simple_primary_key.csv")
     assert response.status == 200
     assert not response.headers.get("Access-Control-Allow-Origin")
-    assert 'text/plain; charset=utf-8' == response.headers['Content-Type']
+    assert "text/plain; charset=utf-8" == response.headers["Content-Type"]
     assert EXPECTED_TABLE_CSV == response.text
 
 
 def test_table_csv_cors_headers(app_client_with_cors):
-    response = app_client_with_cors.get('/fixtures/simple_primary_key.csv')
+    response = app_client_with_cors.get("/fixtures/simple_primary_key.csv")
     assert response.status == 200
     assert "*" == response.headers["Access-Control-Allow-Origin"]
 
 
 def test_table_csv_with_labels(app_client):
-    response = app_client.get('/fixtures/facetable.csv?_labels=1')
+    response = app_client.get("/fixtures/facetable.csv?_labels=1")
     assert response.status == 200
-    assert 'text/plain; charset=utf-8' == response.headers['Content-Type']
+    assert "text/plain; charset=utf-8" == response.headers["Content-Type"]
     assert EXPECTED_TABLE_WITH_LABELS_CSV == response.text
 
 
 def test_custom_sql_csv(app_client):
     response = app_client.get(
-        '/fixtures.csv?sql=select+content+from+simple_primary_key+limit+2'
+        "/fixtures.csv?sql=select+content+from+simple_primary_key+limit+2"
     )
     assert response.status == 200
-    assert 'text/plain; charset=utf-8' == response.headers['Content-Type']
+    assert "text/plain; charset=utf-8" == response.headers["Content-Type"]
     assert EXPECTED_CUSTOM_CSV == response.text
 
 
 def test_table_csv_download(app_client):
-    response = app_client.get('/fixtures/simple_primary_key.csv?_dl=1')
+    response = app_client.get("/fixtures/simple_primary_key.csv?_dl=1")
     assert response.status == 200
-    assert 'text/csv; charset=utf-8' == response.headers['Content-Type']
+    assert "text/csv; charset=utf-8" == response.headers["Content-Type"]
     expected_disposition = 'attachment; filename="simple_primary_key.csv"'
-    assert expected_disposition == response.headers['Content-Disposition']
+    assert expected_disposition == response.headers["Content-Disposition"]
 
 
 def test_max_csv_mb(app_client_csv_max_mb_one):
@@ -88,12 +94,8 @@ def test_max_csv_mb(app_client_csv_max_mb_one):
 
 def test_table_csv_stream(app_client):
     # Without _stream should return header + 100 rows:
-    response = app_client.get(
-        "/fixtures/compound_three_primary_keys.csv?_size=max"
-    )
+    response = app_client.get("/fixtures/compound_three_primary_keys.csv?_size=max")
     assert 101 == len([b for b in response.body.split(b"\r\n") if b])
     # With _stream=1 should return header + 1001 rows
-    response = app_client.get(
-        "/fixtures/compound_three_primary_keys.csv?_stream=1"
-    )
+    response = app_client.get("/fixtures/compound_three_primary_keys.csv?_stream=1")
     assert 1002 == len([b for b in response.body.split(b"\r\n") if b])