Python2 compatible

[arvindkhadri]

Hi,
I am trying to use this project on python2 and I get the following error,

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/sqlalchemy_filters/__init__.py", line 3, in <module>
    from .filters import apply_filters  # noqa: F401
  File "/usr/lib/python2.7/site-packages/sqlalchemy_filters/filters.py", line 3, in <module>
    from inspect import signature
ImportError: cannot import name signature

[mattbennett]

We chose not to support Python 2 for this library, sorry.

inspect.signature is not available before Python 3.3. You may be able to work around it in a fork if you can't upgrade to Python 3.

[arvindkhadri]

I have a work around, I was wondering if I should send a pull request.