Format the codebase using Black (#2109)

This commit:
1. Formats all of our existing code using `black`.
2. Adds a note about using `black` to `CONTRIBUTING.rst`.
3. Runs `black --check` as part of CI (failing builds that aren't properly formatted).

setup.py

@@ -8,13 +8,10 @@ try:
 except ImportError:
     pass
 
-DESCRIPTION = (
-    'MongoEngine is a Python Object-Document '
-    'Mapper for working with MongoDB.'
-)
+DESCRIPTION = "MongoEngine is a Python Object-Document Mapper for working with MongoDB."
 
 try:
-    with open('README.rst') as fin:
+    with open("README.rst") as fin:
         LONG_DESCRIPTION = fin.read()
 except Exception:
     LONG_DESCRIPTION = None
@@ -24,23 +21,23 @@ def get_version(version_tuple):
     """Return the version tuple as a string, e.g. for (0, 10, 7),
     return '0.10.7'.
     """
-    return '.'.join(map(str, version_tuple))
+    return ".".join(map(str, version_tuple))
 
 
 # Dirty hack to get version number from monogengine/__init__.py - we can't
 # import it as it depends on PyMongo and PyMongo isn't installed until this
 # file is read
-init = os.path.join(os.path.dirname(__file__), 'mongoengine', '__init__.py')
-version_line = list(filter(lambda l: l.startswith('VERSION'), open(init)))[0]
+init = os.path.join(os.path.dirname(__file__), "mongoengine", "__init__.py")
+version_line = list(filter(lambda l: l.startswith("VERSION"), open(init)))[0]
 
-VERSION = get_version(eval(version_line.split('=')[-1]))
+VERSION = get_version(eval(version_line.split("=")[-1]))
 
 CLASSIFIERS = [
-    'Development Status :: 4 - Beta',
-    'Intended Audience :: Developers',
-    'License :: OSI Approved :: MIT License',
-    'Operating System :: OS Independent',
-    'Programming Language :: Python',
+    "Development Status :: 4 - Beta",
+    "Intended Audience :: Developers",
+    "License :: OSI Approved :: MIT License",
+    "Operating System :: OS Independent",
+    "Programming Language :: Python",
     "Programming Language :: Python :: 2",
     "Programming Language :: Python :: 2.7",
     "Programming Language :: Python :: 3",
@@ -48,39 +45,40 @@ CLASSIFIERS = [
     "Programming Language :: Python :: 3.6",
     "Programming Language :: Python :: Implementation :: CPython",
     "Programming Language :: Python :: Implementation :: PyPy",
-    'Topic :: Database',
-    'Topic :: Software Development :: Libraries :: Python Modules',
+    "Topic :: Database",
+    "Topic :: Software Development :: Libraries :: Python Modules",
 ]
 
 extra_opts = {
-    'packages': find_packages(exclude=['tests', 'tests.*']),
-    'tests_require': ['nose', 'coverage==4.2', 'blinker', 'Pillow>=2.0.0']
+    "packages": find_packages(exclude=["tests", "tests.*"]),
+    "tests_require": ["nose", "coverage==4.2", "blinker", "Pillow>=2.0.0"],
 }
 if sys.version_info[0] == 3:
-    extra_opts['use_2to3'] = True
-    if 'test' in sys.argv or 'nosetests' in sys.argv:
-        extra_opts['packages'] = find_packages()
-        extra_opts['package_data'] = {
-            'tests': ['fields/mongoengine.png', 'fields/mongodb_leaf.png']}
+    extra_opts["use_2to3"] = True
+    if "test" in sys.argv or "nosetests" in sys.argv:
+        extra_opts["packages"] = find_packages()
+        extra_opts["package_data"] = {
+            "tests": ["fields/mongoengine.png", "fields/mongodb_leaf.png"]
+        }
 else:
-    extra_opts['tests_require'] += ['python-dateutil']
+    extra_opts["tests_require"] += ["python-dateutil"]
 
 setup(
-    name='mongoengine',
+    name="mongoengine",
     version=VERSION,
-    author='Harry Marr',
-    author_email='harry.marr@gmail.com',
+    author="Harry Marr",
+    author_email="harry.marr@gmail.com",
     maintainer="Stefan Wojcik",
     maintainer_email="wojcikstefan@gmail.com",
-    url='http://mongoengine.org/',
-    download_url='https://github.com/MongoEngine/mongoengine/tarball/master',
-    license='MIT',
+    url="http://mongoengine.org/",
+    download_url="https://github.com/MongoEngine/mongoengine/tarball/master",
+    license="MIT",
     include_package_data=True,
     description=DESCRIPTION,
     long_description=LONG_DESCRIPTION,
-    platforms=['any'],
+    platforms=["any"],
     classifiers=CLASSIFIERS,
-    install_requires=['pymongo>=3.4', 'six'],
-    test_suite='nose.collector',
+    install_requires=["pymongo>=3.4", "six"],
+    test_suite="nose.collector",
     **extra_opts
 )