Add support for functions with type annotations

[abustany]

It looks like the API used by Flask-JsonRPC to inspect functions (as of version 0.3.1) is not compatible with typing annotations...

Example:

@jsonrpc.method('Test.Ping')
def ping() -> str:
    return 'pong'

causes the following exception:

Traceback (most recent call last):
  File "./test.py", line 17, in <module>
    class Server:
  File "./test.py", line 29, in Server
    def ping() -> str:
  File "/home/abustany/test/.direnv/python-3.7.1/lib/python3.7/site-packages/flask_jsonrpc/__init__.py", line 225, in decorator
    arg_names = getargspec(f)[0]
  File "/usr/lib64/python3.7/inspect.py", line 1080, in getargspec
    raise ValueError("Function has keyword-only parameters or annotations"
ValueError: Function has keyword-only parameters or annotations, use getfullargspec() API which can support them