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_plugins.py

@@ -1,7 +1,5 @@
 from bs4 import BeautifulSoup as Soup
-from .fixtures import ( # noqa
-    app_client,
-)
+from .fixtures import app_client  # noqa
 import base64
 import json
 import re
@@ -13,41 +11,26 @@ def test_plugins_dir_plugin(app_client):
     response = app_client.get(
         "/fixtures.json?sql=select+convert_units(100%2C+'m'%2C+'ft')"
     )
-    assert pytest.approx(328.0839) == response.json['rows'][0][0]
+    assert pytest.approx(328.0839) == response.json["rows"][0][0]
 
 
 @pytest.mark.parametrize(
     "path,expected_decoded_object",
     [
-        (
-            "/",
-            {
-                "template": "index.html",
-                "database": None,
-                "table": None,
-            },
-        ),
+        ("/", {"template": "index.html", "database": None, "table": None}),
         (
             "/fixtures/",
-            {
-                "template": "database.html",
-                "database": "fixtures",
-                "table": None,
-            },
+            {"template": "database.html", "database": "fixtures", "table": None},
         ),
         (
             "/fixtures/sortable",
-            {
-                "template": "table.html",
-                "database": "fixtures",
-                "table": "sortable",
-            },
+            {"template": "table.html", "database": "fixtures", "table": "sortable"},
         ),
     ],
 )
 def test_plugin_extra_css_urls(app_client, path, expected_decoded_object):
     response = app_client.get(path)
-    links = Soup(response.body, 'html.parser').findAll('link')
+    links = Soup(response.body, "html.parser").findAll("link")
     special_href = [
         l for l in links if l.attrs["href"].endswith("/extra-css-urls-demo.css")
     ][0]["href"]
@@ -59,47 +42,43 @@ def test_plugin_extra_css_urls(app_client, path, expected_decoded_object):
 
 
 def test_plugin_extra_js_urls(app_client):
-    response = app_client.get('/')
-    scripts = Soup(response.body, 'html.parser').findAll('script')
+    response = app_client.get("/")
+    scripts = Soup(response.body, "html.parser").findAll("script")
     assert [
-        s for s in scripts
-        if s.attrs == {
-            'integrity': 'SRIHASH',
-            'crossorigin': 'anonymous',
-            'src': 'https://example.com/jquery.js'
+        s
+        for s in scripts
+        if s.attrs
+        == {
+            "integrity": "SRIHASH",
+            "crossorigin": "anonymous",
+            "src": "https://example.com/jquery.js",
         }
     ]
 
 
 def test_plugins_with_duplicate_js_urls(app_client):
     # If two plugins both require jQuery, jQuery should be loaded only once
-    response = app_client.get(
-        "/fixtures"
-    )
+    response = app_client.get("/fixtures")
     # This test is a little tricky, as if the user has any other plugins in
     # their current virtual environment those may affect what comes back too.
     # What matters is that https://example.com/jquery.js is only there once
     # and it comes before plugin1.js and plugin2.js which could be in either
     # order
-    scripts = Soup(response.body, 'html.parser').findAll('script')
-    srcs = [s['src'] for s in scripts if s.get('src')]
+    scripts = Soup(response.body, "html.parser").findAll("script")
+    srcs = [s["src"] for s in scripts if s.get("src")]
     # No duplicates allowed:
     assert len(srcs) == len(set(srcs))
     # jquery.js loaded once:
-    assert 1 == srcs.count('https://example.com/jquery.js')
+    assert 1 == srcs.count("https://example.com/jquery.js")
     # plugin1.js and plugin2.js are both there:
-    assert 1 == srcs.count('https://example.com/plugin1.js')
-    assert 1 == srcs.count('https://example.com/plugin2.js')
+    assert 1 == srcs.count("https://example.com/plugin1.js")
+    assert 1 == srcs.count("https://example.com/plugin2.js")
     # jquery comes before them both
-    assert srcs.index(
-        'https://example.com/jquery.js'
-    ) < srcs.index(
-        'https://example.com/plugin1.js'
+    assert srcs.index("https://example.com/jquery.js") < srcs.index(
+        "https://example.com/plugin1.js"
     )
-    assert srcs.index(
-        'https://example.com/jquery.js'
-    ) < srcs.index(
-        'https://example.com/plugin2.js'
+    assert srcs.index("https://example.com/jquery.js") < srcs.index(
+        "https://example.com/plugin2.js"
     )
 
 
@@ -107,13 +86,9 @@ def test_plugins_render_cell_link_from_json(app_client):
     sql = """
         select '{"href": "http://example.com/", "label":"Example"}'
     """.strip()
-    path = "/fixtures?" + urllib.parse.urlencode({
-        "sql": sql,
-    })
+    path = "/fixtures?" + urllib.parse.urlencode({"sql": sql})
     response = app_client.get(path)
-    td = Soup(
-        response.body, "html.parser"
-    ).find("table").find("tbody").find("td")
+    td = Soup(response.body, "html.parser").find("table").find("tbody").find("td")
     a = td.find("a")
     assert a is not None, str(a)
     assert a.attrs["href"] == "http://example.com/"
@@ -129,10 +104,7 @@ def test_plugins_render_cell_demo(app_client):
         "column": "content",
         "table": "simple_primary_key",
         "database": "fixtures",
-        "config": {
-            "depth": "table",
-            "special": "this-is-simple_primary_key"
-        }
+        "config": {"depth": "table", "special": "this-is-simple_primary_key"},
     } == json.loads(td.string)