crs.py: remove @abstractmethod decorator

[hemberger]

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.

• Tests added
• Fully documented, including history.rst for all changes and api/*.rst for new API

[hemberger]

A couple notes about this PR:

1. Even though the python interpreter ignores @abstractmethod here, it can still cause a problem for linters. For example:

   import abc 
   import pyproj.crs
   
   class Foo(pyproj.crs.CustomConstructorCRS, metaclass=abc.ABCMeta):
       pass
   
   Foo()

   This runs properly, but pylint will report the following error:

   $ pylint --errors-only test.py
   ************* Module test
   test.py:6:0: E0110: Abstract class 'Foo' with abstract methods instantiated (abstract-class-instantiated)
   

   While this is a false positive that might be fixable in pylint, it is still a symptom of @abstractmethod being used improperly.

2. An alternative solution would be to make CustomConstructorCRS an abstract class, but this seems too heavy-handed since it would break backwards compatibility for any class that derived from CustomConstructorCRS, but which has not implemented the _expected_types method (which has an entirely reasonable implementation in the CustomConstructorCRS class). This is the case with, for example, the CRS wrapper class in the cartopy module.

Please let me know if any changes need to be made to this PR to make it suitable for merging. Thank you for your time!

[snowman2]

Thanks @hemberger 👍