docmnets fails on class constructors

[muellerzr]

If we pass in a class for the docments function, it completely fails. I would expect it to look at the __init__ for all the relevant information it would need (as it's the class constructor). Basic breaking example:

class Add:
    "A basic addition class"
    def __init__(
        self,
        a:int = 0, # The first number to be added
        b:(int, float) = 1.5, # The second number to be added
    ):
        self.a = a
        self.b = b
    
    def add(
        self,
    ) -> Union[int, float]:
        "Adds `self.a` and `self.b`"
        return self.a + self.b

docments(Add)

Stack trace:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-101-f7728d42a6d5> in <module>()
----> 1 docments(Add)

/mnt/d/lib/python3.7/site-packages/fastcore/docments.py in docments(s, full, returns)
     55 def docments(s, full=False, returns=True):
     56     "`dict` of parameter names to 'docment-style' comments in function or string `s`"
---> 57     comments = {o.start[0]:_clean_comment(o.string) for o in _tokens(s) if o.type==COMMENT}
     58     parms = _param_locs(s, returns=returns)
     59     docs = {arg:_get_comment(line, arg, comments, parms) for line,arg in parms.items()}

/mnt/d/lib/python3.7/site-packages/fastcore/docments.py in _tokens(s)
     22     "Tokenize Python code in string or function object `s`"
     23     if isfunction(s): s = getsource(s)
---> 24     return tokenize(BytesIO(s.encode('utf-8')).readline)
     25 
     26 _clean_re = re.compile('^\s*#(.*)\s*$')

AttributeError: type object 'Add' has no attribute 'encode'

[muellerzr]

A potential fix is changing the first line of docments to see if it's a class, and if so call the __init__

    if isclass(s): s = s.__init__ # Constructor for a class
    comments = {o.start[0]:_clean_comment(o.string) for o in _tokens(s) if o.type==COMMENT}
    ...

[muellerzr]

Fixed with #347