Makefun

* 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).

* Fixed `NameError` in case of unknown symbols in type hints. Fixes [37](https://github.com/smarie/python-makefun/issues/37).

* Fixed PEP8 error in source code. Fixes [35](https://github.com/smarie/python-makefun/issues/35).

* Now string signatures can contain a colon. Fixes [36](https://github.com/smarie/python-makefun/issues/36)

Fixed bug when the return type annotation of the function to create contains non-locally available type hints. Fixes [33](https://github.com/smarie/python-makefun/issues/33).

Fixed bug when the signature of the function to create contains non-locally available type hints. Fixes [32](https://github.com/smarie/python-makefun/issues/32).