Pretty print function signatures

[lgeiger]

This PR auto-formats function signatures. This is makes very long function signatures a lot more readable.

Before

SeparableConv2D(filters, kernel_size, strides=(1, 1), padding='valid', data_format=None, dilation_rate=(1, 1), depth_multiplier=1, activation=None, use_bias=True, depthwise_initializer='glorot_uniform', pointwise_initializer='glorot_uniform', bias_initializer='zeros', depthwise_regularizer=None, pointwise_regularizer=None, bias_regularizer=None, activity_regularizer=None, depthwise_constraint=None, pointwise_constraint=None, bias_constraint=None, **kwargs)

After

SeparableConv2D(
    filters,
    kernel_size,
    strides=(1, 1),
    padding='valid',
    data_format=None,
    dilation_rate=(1, 1),
    depth_multiplier=1,
    activation=None,
    use_bias=True,
    depthwise_initializer='glorot_uniform',
    pointwise_initializer='glorot_uniform',
    bias_initializer='zeros',
    depthwise_regularizer=None,
    pointwise_regularizer=None,
    bias_regularizer=None,
    activity_regularizer=None,
    depthwise_constraint=None,
    pointwise_constraint=None,
    bias_constraint=None,
    **kwargs)

Why using yapf?

• black doesn't yet have a stable API: Black's public API psf/black#779
• autopep8 had weird whitespace handling with tuple arguments

[NiklasRosenstein]

Thanks, looks cool! :-)

[florimondmanca]

Hi, I just ran into failing builds, apparently caused by yapf not supporting type annotations:

$ pydocmd generate
/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/pydocmd/__main__.py:47: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
  config = yaml.load(fp)
Started copying source files...
Generating temporary MkDocs config...
Building index...
Started generating documentation...

Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/yapf/yapflib/pytree_utils.py", line 115, in ParseCodeToTree
    tree = parser_driver.parse_string(code, debug=False)
  File "/opt/python/3.6.3/lib/python3.6/lib2to3/pgen2/driver.py", line 106, in parse_string
    return self.parse_tokens(tokens, debug)
  File "/opt/python/3.6.3/lib/python3.6/lib2to3/pgen2/driver.py", line 71, in parse_tokens
    if p.addtoken(type, value, (prefix, start)):
  File "/opt/python/3.6.3/lib/python3.6/lib2to3/pgen2/parse.py", line 159, in addtoken
    raise ParseError("bad input", type, value, context)
lib2to3.pgen2.parse.ParseError: bad input: type=11, value=':', context=('', (1, 16))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/yapf/yapflib/pytree_utils.py", line 121, in ParseCodeToTree
    tree = parser_driver.parse_string(code, debug=False)
  File "/opt/python/3.6.3/lib/python3.6/lib2to3/pgen2/driver.py", line 106, in parse_string
    return self.parse_tokens(tokens, debug)
  File "/opt/python/3.6.3/lib/python3.6/lib2to3/pgen2/driver.py", line 71, in parse_tokens
    if p.addtoken(type, value, (prefix, start)):
  File "/opt/python/3.6.3/lib/python3.6/lib2to3/pgen2/parse.py", line 159, in addtoken
    raise ParseError("bad input", type, value, context)
lib2to3.pgen2.parse.ParseError: bad input: type=11, value=':', context=('', (1, 16))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.6.3/bin/pydocmd", line 11, in <module>
    load_entry_point('pydoc-markdown==2.0.5', 'console_scripts', 'pydocmd')()
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/pydocmd/__main__.py", line 252, in main
    loader.load_section(section)
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/pydocmd/loader.py", line 92, in load_section
    sig, _ = FormatCode(sig, style_config='pep8')
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/yapf/yapflib/yapf_api.py", line 129, in FormatCode
    tree = pytree_utils.ParseCodeToTree(unformatted_source)
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/yapf/yapflib/pytree_utils.py", line 127, in ParseCodeToTree
    raise e
  File "/home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages/yapf/yapflib/pytree_utils.py", line 125, in ParseCodeToTree
    ast.parse(code)
  File "/opt/python/3.6.3/lib/python3.6/ast.py", line 35, in parse
    return compile(source, filename, mode, PyCF_ONLY_AST)
  File "<unknown>", line 1
    App.mount(prefix:str, app:Union[_ForwardRef('App'), bocadillo.app_types.ASGIApp, Callable[[dict, Callable[[str, List[str]], NoneType]], List[bytes]]])
                    ^
SyntaxError: invalid syntax
The command "pydocmd generate" exited with 1.

I had to pin to <2.0.5 to have my builds run again. :-)

Full failing build traceback: https://travis-ci.org/bocadilloproject/bocadillo/builds/515720068