Incompatible with f strings in Python 3.6

As the title says, the current version is incompatible with [f strings in Python 3.6](https://www.python.org/dev/peps/pep-0498/).

```
>>> print(Color("{autored}hi{/red}"))
hi
>>> print(Color(f"{autored}hi{/red}"))
  File "<fstring>", line 1
    (/red)
     ^
SyntaxError: invalid syntax
```