``funcsigs`` is a backport of the `PEP 362`_ function signature features from
Python 3.3's `inspect`_ module. The backport is compatible with Python 2.6, 2.7
as well as 3.3 and up. 3.2
There is only one place which is used in pip-shims, but the execption
will never happen on current supported versions of pipenv and Python.
https://github.com/sarugaku/pip-shims/blob/d165d55a53065d51aaaebde4124a98c6a8c9fcba/src/pip_shims/utils.py#L387
```
    try:
        signature = inspect.signature(fn_or_class)
    except AttributeError: # never reached on python 3.3 or later.
        import funcsigs
```