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.

setup.py

@@ -1,72 +1,78 @@
 from setuptools import setup, find_packages
 import os
+import sys
 
 import versioneer
 
 
 def get_long_description():
-    with open(os.path.join(
-        os.path.dirname(os.path.abspath(__file__)), 'README.md'
-    ), encoding='utf8') as fp:
+    with open(
+        os.path.join(os.path.dirname(os.path.abspath(__file__)), "README.md"),
+        encoding="utf8",
+    ) as fp:
         return fp.read()
 
 
 def get_version():
     path = os.path.join(
-        os.path.dirname(os.path.abspath(__file__)), 'datasette', 'version.py'
+        os.path.dirname(os.path.abspath(__file__)), "datasette", "version.py"
     )
     g = {}
     exec(open(path).read(), g)
-    return g['__version__']
+    return g["__version__"]
 
 
+# Only install black on Python 3.6 or higher
+maybe_black = []
+if sys.version_info > (3, 6):
+    maybe_black = ["black"]
+
 setup(
-    name='datasette',
+    name="datasette",
     version=versioneer.get_version(),
     cmdclass=versioneer.get_cmdclass(),
-    description='An instant JSON API for your SQLite databases',
+    description="An instant JSON API for your SQLite databases",
     long_description=get_long_description(),
-    long_description_content_type='text/markdown',
-    author='Simon Willison',
-    license='Apache License, Version 2.0',
-    url='https://github.com/simonw/datasette',
+    long_description_content_type="text/markdown",
+    author="Simon Willison",
+    license="Apache License, Version 2.0",
+    url="https://github.com/simonw/datasette",
     packages=find_packages(),
-    package_data={'datasette': ['templates/*.html']},
+    package_data={"datasette": ["templates/*.html"]},
     include_package_data=True,
     install_requires=[
-        'click>=6.7',
-        'click-default-group==1.2',
-        'Sanic==0.7.0',
-        'Jinja2==2.10.1',
-        'hupper==1.0',
-        'pint==0.8.1',
-        'pluggy>=0.7.1',
+        "click>=6.7",
+        "click-default-group==1.2",
+        "Sanic==0.7.0",
+        "Jinja2==2.10.1",
+        "hupper==1.0",
+        "pint==0.8.1",
+        "pluggy>=0.7.1",
     ],
-    entry_points='''
+    entry_points="""
         [console_scripts]
         datasette=datasette.cli:cli
-    ''',
-    setup_requires=['pytest-runner'],
+    """,
+    setup_requires=["pytest-runner"],
     extras_require={
-        'test': [
-            'pytest==4.0.2',
-            'pytest-asyncio==0.10.0',
-            'aiohttp==3.5.3',
-            'beautifulsoup4==4.6.1',
+        "test": [
+            "pytest==4.0.2",
+            "pytest-asyncio==0.10.0",
+            "aiohttp==3.5.3",
+            "beautifulsoup4==4.6.1",
         ]
+        + maybe_black
     },
-    tests_require=[
-        'datasette[test]',
-    ],
+    tests_require=["datasette[test]"],
     classifiers=[
-        'Development Status :: 4 - Beta',
-        'Intended Audience :: Developers',
-        'Intended Audience :: Science/Research',
-        'Intended Audience :: End Users/Desktop',
-        'Topic :: Database',
-        'License :: OSI Approved :: Apache Software License',
-        'Programming Language :: Python :: 3.7',
-        'Programming Language :: Python :: 3.6',
-        'Programming Language :: Python :: 3.5',
+        "Development Status :: 4 - Beta",
+        "Intended Audience :: Developers",
+        "Intended Audience :: Science/Research",
+        "Intended Audience :: End Users/Desktop",
+        "Topic :: Database",
+        "License :: OSI Approved :: Apache Software License",
+        "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.6",
+        "Programming Language :: Python :: 3.5",
     ],
 )