Makefun

`add_signature_parameters` now accepts that one specifies a custom index where to insert the new parameters.

Added `__version__` attribute to comply with PEP396, following [this guide](https://smarie.github.io/python-getversion/#package-versioning-best-practices). Fixes [45](https://github.com/smarie/python-makefun/issues/45).

Fixed `six` dependency: also declared as a setup dependency.

Added missing `six` dependency explicitly.

* Improved performance of inner method `get_signature_string` (used by all entry points) after profiling.

1.6.7 - Increased tolerance to function signatures in python 2

* In python 2 some libraries such as `attrs` can modify the annotations manually, making `signature` return a string representation that is not compliant with the language version. This raised a `SyntaxError` in previous versions. The new version silently removes all these annotations in python versions that do not support them. Fixes [39](https://github.com/smarie/python-makefun/issues/39).

* Fixed yet another nasty varpositional-related bug :). Fixes [38](https://github.com/smarie/python-makefun/issues/38).