crs.py: remove @abstractmethod decorator
crs.py: remove @abstractmethod decorator

According to the documentation for the `@abstractmethod` decorator:

> Using this decorator requires that the class's metaclass is
> ABCMeta or is derived from it.

(source: https://docs.python.org/3/library/abc.html#abc.abstractmethod)

Since `CustomConstructorCRS` and `CRS` are not defined as abstract
classes, using the `@abstractmethod` decorator does not actually do
anything and should be removed.

ocefpaf/pyproj,ocefpaf/pyproj,pyproj4/pyproj,pyproj4/pyproj